Light-emitting element, light-emitting device, electronic appliance, and lighting device

ABSTRACT

A light-emitting element which includes a plurality of light-emitting layers between a pair of electrodes and has low driving voltage and high emission efficiency is provided. A light-emitting element including first to third light-emitting layers between a cathode and an anode is provided. The first light-emitting layer includes a first phosphorescent material and a first electron-transport material; the second light-emitting layer includes a second phosphorescent material and a second electron-transport material; the third light-emitting layer includes a fluorescent material and a third electron-transport material; the first to third light-emitting elements are provided in contact with an electron-transport layer positioned on a cathode side; and a triplet excitation energy level of a material included in the electron-transport layer is lower than triplet excitation energy levels of the first electron-transport material and the second electron-transport material.

This application is a continuation of copending U.S. application Ser.No. 15/882,178, filed on Jan. 29, 2018 which is a continuation of U.S.application Ser. No. 14/226,360, filed on Mar. 26, 2014 (now U.S. Pat.No. 9,893,303 issued Feb. 13, 2018) which are all incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a light-emittingelement in which a light-emitting layer capable of emitting light byapplication of an electric field is provided between a pair ofelectrodes, and also relates to a light-emitting device and a lightingdevice including such a light-emitting element.

2. Description of the Related Art

Light-emitting elements which include organic compounds as alight-emitting layer and are characterized by thinness, lightweight,fast response, and direct current driving with low voltage are expectedto be applied to next-generation flat panel displays. In particular, adisplay device in which light-emitting elements are arranged in matrixis considered to have advantages in a wide viewing angle and excellentvisibility over a conventional liquid crystal display device.

It is said that a light-emitting element has the following lightemission mechanism: when voltage is applied between a pair of electrodeswith a light-emitting layer including a luminous body providedtherebetween, electrons injected from the cathode and holes injectedfrom the anode are recombined in an light emission center of thelight-emitting layer to form molecular excitons, and energy is releasedand light is emitted when the molecular excitons relax to the groundstate. A singlet excited state and a triplet excited state are known asexcited states, and light emission can probably be obtained througheither state. Light emission from the singlet excited state (S*) iscalled fluorescence, and light emission from the triplet excited state(T*) is called phosphorescence.

In order to improve element characteristics or productivity of suchlight-emitting elements, improvement of an element structure,development of a material, and the like have been actively carried out.Further, research and development have been extensively conducted onorganic EL elements as light-emitting elements, and full-color organicEL elements have been actively developed.

As a way to achieve full-color display, for example, light-emittinglayers of pixels are separately deposited. The light-emitting layers aredeposited on necessary pixels using a shadow mask. In this case, toreduce cost by reducing the number of steps, Patent Document 1 disclosesa structure in which layers except a light-emitting layer, for example,a hole-transport layer, an electron-transport layer, and a cathode areformed to be shared by a plurality of pixels.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2004-006362

SUMMARY OF THE INVENTION

In the case of the structure disclosed in Patent Document 1, since thehole-transport layer or the electron-transport layer is used to beshared by the plurality of pixels, pixels emitting different colorsdiffer in element characteristics such as driving voltage. Further, inthe case of such a structure, the hole-transport layer or theelectron-transport layer is shared by pixels emitting different colors;therefore, each pixel does not have an optimal structure, which causes aproblem in that an abnormal state of the element characteristics, e.g.,an increase in driving voltage or a reduction in reliability, occurs inat least one of the plurality of pixels.

In view of the above problems, an object of one embodiment of thepresent invention is to provide a light-emitting element which includesa plurality of light-emitting layers between a pair of electrodes andhas low driving voltage and high emission efficiency.

One embodiment of the present invention is a light-emitting elementincluding first to third light-emitting layers between a cathode and ananode. The first light-emitting layer includes a first phosphorescentmaterial and a first electron-transport material; the secondlight-emitting layer includes a second phosphorescent material and asecond electron-transport material; the third light-emitting layerincludes a fluorescent material and a third electron-transport material;the first to third light-emitting elements are provided in contact withan electron-transport layer positioned on a cathode side; and a tripletexcitation energy level of a material included in the electron-transportlayer is lower than triplet excitation energy levels of the firstelectron-transport material and the second electron-transport material.

Such a structure in which the electron-transport layer is in contactwith the first to third light-emitting layers can increase productivityat the time of forming the light-emitting element. Further, theelectron-transport layer is formed using a material whose tripletexcitation energy level (T1 level) is lower than those of the firstelectron-transport material and the second electron-transport material.Since the first electron-transport material and the secondelectron-transport material have a high electron-transport property, thelight-emission region of the light-emitting element of one embodiment ofthe present invention is formed in the light-emitting layer on thehole-transport layer side. Therefore, the first light-emitting layer andthe second light-emitting layer are not affected by the low T1 level ofthe electron-transport layer, and thus, an element structure whichenables low driving voltage and high emission efficiency can beobtained.

Another embodiment of the present invention is a light-emitting elementincluding first to third light-emitting layers between a cathode and ananode. The first light-emitting layer includes a first phosphorescentmaterial and a first electron-transport material; the secondlight-emitting layer includes a second phosphorescent material and asecond electron-transport material; the third light-emitting layerincludes a fluorescent material and a third electron-transport materialand is in contact with the first and second light-emitting layers on acathode side; and a triplet excitation energy level of the thirdelectron-transport material is lower than triplet excitation energylevels of the first electron-transport material and the secondelectron-transport material.

When the third light-emitting layer is provided in contact with thefirst light-emitting layer and the second light-emitting layer on thecathode side in the above manner, the third light-emitting layerfunctions as an electron-transport layer over the first light-emittinglayer and the second light-emitting layer and functions as alight-emitting layer in the third light-emitting layer. Note that thefluorescent material (also referred to as a dopant or a guest material)included in the third light-emitting layer does not contribute to lightemission over the first light-emitting layer and the secondlight-emitting layer because the first electron-transport material andthe second electron-transport material have a high electron-transportproperty. On the other hand, light emission can be obtained from thefluorescent material in the third light-emitting layer. That is, thethird light-emitting layer has a function of an electron-transport layerand a function of a light-emitting layer at the same time; therefore,the third light-emitting layer can be used as an electron-transportlayer to be shared over the first light-emitting layer and the secondlight-emitting layer, and can be used as a light-emitting layer in thethird light-emitting layer. Thus, the productivity at the time offorming the light-emitting element can be increased.

A light-emitting device including the light-emitting element and anelectronic device and a lighting device each including thelight-emitting device are also included in the scope of one embodimentof the present invention. Therefore, the light-emitting device in thisspecification refers to an image display device, or a light source(including a lighting device). In addition, the light-emitting deviceincludes, in its category, all of a module in which a light-emittingdevice is connected to a connector such as a flexible printed circuit(FPC), a tape carrier package (TCP), a module in which a printed wiringboard is provided on the tip of a TCP, and a module in which anintegrated circuit (IC) is directly mounted on a light-emitting elementby a chip on glass (COG) method.

A light-emitting element which includes a plurality of light-emittinglayers between a pair of electrodes and has low driving voltage and highemission efficiency can be provided as a light-emitting element of oneembodiment present invention. Further, productivity can be improved atthe time of forming the light-emitting element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate a light-emitting element of one embodiment ofthe present invention.

FIGS. 2A and 2B each illustrate a light-emitting element of oneembodiment of the present invention.

FIGS. 3A and 3B each illustrate a light-emitting element of oneembodiment of the present invention.

FIGS. 4A and 4B each illustrate a light-emitting device including alight-emitting element of one embodiment of the present invention.

FIGS. 5A to 5E illustrate electronic appliances including alight-emitting element and a light-emitting device of one embodiment ofthe present invention.

FIGS. 6A and 6B illustrate light-emitting elements of Examples.

FIG. 7 is a graph showing current density-luminance characteristics of alight-emitting element 1 and a comparative light-emitting element 2.

FIG. 8 is a graph showing voltage-luminance characteristics of alight-emitting element 1 and a comparative light-emitting element 2.

FIG. 9 is a graph showing luminance-current efficiency characteristicsof a light-emitting element 1 and a comparative light-emitting element2.

FIG. 10 is a graph showing voltage-current characteristics of alight-emitting element 1 and a comparative light-emitting element 2.

FIG. 11 is a graph showing emission spectra of a light-emitting element1 and a comparative light-emitting element 2.

FIG. 12 is a graph showing current density-luminance characteristics ofa light-emitting element 3 and a comparative light-emitting element 4.

FIG. 13 is a graph showing voltage-luminance characteristics of alight-emitting element 3 and a comparative light-emitting element 4.

FIG. 14 is a graph showing luminance-current efficiency characteristicsof a light-emitting element 3 and a comparative light-emitting element4.

FIG. 15 is a graph showing voltage-current characteristics of alight-emitting element 3 and a comparative light-emitting element 4.

FIG. 16 is a graph showing emission spectra of a light-emitting element3 and a comparative light-emitting element 4.

FIG. 17 is a graph showing current density-luminance characteristics ofa light-emitting element 5 and a comparative light-emitting element 6.

FIG. 18 is a graph showing voltage-luminance characteristics of alight-emitting element 5 and a comparative light-emitting element 6.

FIG. 19 is a graph showing luminance-current efficiency characteristicsof a light-emitting element 5 and a comparative light-emitting element6.

FIG. 20 is a graph showing voltage-current characteristics of alight-emitting element 5 and a comparative light-emitting element 6.

FIG. 21 is a graph showing emission spectra of a light-emitting element5 and a comparative light-emitting element 6.

FIG. 22 is a graph showing current density-luminance characteristics oflight-emitting elements 7 and 8.

FIG. 23 is a graph showing voltage-luminance characteristics oflight-emitting elements 7 and 8.

FIG. 24 is a graph showing luminance-current efficiency characteristicsof light-emitting elements 7 and 8.

FIG. 25 is a graph showing voltage-current characteristics oflight-emitting elements 7 and 8.

FIG. 26 is a graph showing emission spectra of light-emitting elements 7and 8.

FIG. 27A is a graph showing results of reliability tests oflight-emitting elements 1 and 7, and a comparative light-emittingelement 2, and FIG. 27B is a graph showing results of reliability testsof light-emitting elements 3 and 8, and a comparative light-emittingelement 4.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments and examples of the present invention will be explainedbelow with reference to the drawings. However, the present invention isnot limited to description to be given below, and it is to be easilyunderstood that modes and details thereof can be variously modifiedwithout departing from the purpose and the scope of the presentinvention. Accordingly, the present invention should not be interpretedas being limited to the content of the embodiments and examples below.

Note that the position, the size, the range, or the like of eachstructure illustrated in drawings and the like is not accuratelyrepresented in some cases for simplification. Therefore, the disclosedinvention is not necessarily limited to the position, the size, therange, or the like disclosed in the drawings and the like.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not mean limitation of the number ofcomponents.

Embodiment 1

In this embodiment, a structural concept of a light-emitting element ofone embodiment of the present invention and a specific structure of thelight-emitting element are described. First, the light-emitting elementthat is one embodiment of the present invention is described withreference to FIGS. 1A and 1B.

A light-emitting element illustrated in FIG. 1A includes alight-emitting layer 115 between a pair of electrodes (an anode 101 anda cathode 103). The light-emitting layer 115 includes a firstlight-emitting layer 115 a including a first phosphorescent material 121a and a first electron-transport material 122 a; a second light-emittinglayer 115 b including a second phosphorescent material 131 a and asecond electron-transport material 132 a; and a third light-emittinglayer 115 c including a fluorescent material 141 a and a thirdelectron-transport material 142 a.

The first light-emitting layer 115 a, the second light-emitting layer115 b, and the third light-emitting layer 115 c are each in contact withan electron-transport layer 117 which is positioned on the cathode 103side.

The first light-emitting layer 115 a may further include a firsthole-transport material 123 a, in addition to the first phosphorescentmaterial 121 a and the first electron-transport material 122 a. Thesecond light-emitting layer 115 b may further include a secondhole-transport material 133 a, in addition to the second phosphorescentmaterial 131 a and the second electron-transport material 132 a.

In the first light-emitting layer 115 a, the first electron-transportmaterial 122 a functions as a host material, and the firstphosphorescent material 121 a functions as a guest material (alsoreferred to as a dopant). The first hole-transport material 123 afunctions as an assist material. That is, the first phosphorescentmaterial 121 a and the first hole-transport material 123 a are dispersedinto the first electron-transport material 122 a functioning as a hostmaterial. In the second light-emitting layer 115 b, the secondelectron-transport material 132 a functions as a host material, and thesecond phosphorescent material 131 a functions as a guest material. Thesecond hole-transport material 133 a functions as an assist material.That is, the second phosphorescent material 131 a and the secondhole-transport material 133 a are dispersed into the secondelectron-transport material 132 a functioning as a host material. In thethird light-emitting layer 115 c, the third electron-transport material142 a functions as a host material, and the fluorescent material 141 afunctions as a guest material. That is, the fluorescent material 141 ais dispersed into the third electron-transport material 142 afunctioning as a host material.

For example, a red-light-emitting phosphorescent material can be used asa light-emitting substance of the first phosphorescent material 121 a.Further, a green-light-emitting phosphorescent material can be used as alight-emitting substance of the second phosphorescent material 131 a.Furthermore, a blue-light-emitting fluorescent material can be used as alight-emitting substance of the fluorescent material 141 a. Note that inthis specification, the maximum emission wavelength of thered-light-emitting phosphorescent material is longer than 570 nm andshorter than or equal to 740 nm, the maximum emission wavelength of thegreen-light-emitting phosphorescent material is longer than 500 nm andshorter than or equal to 570 nm, and the maximum emission wavelength ofthe blue-light-emitting fluorescent material is longer than or equal to400 nm and shorter than or equal to 500 nm.

In FIG. 1A, in addition to the light-emitting layer 115 and theelectron-transport layer 117, a hole-injection layer 111, a firsthole-transport layer 113 a, a second hole-transport layer 113 b, a thirdhole-transport layer 113 c, and an electron-injection layer 119 areprovided between the pair of electrodes.

Specifically, the light-emitting element illustrated in FIG. 1A includesthe anode 101 over a substrate 100; the hole-injection layer 111 overthe anode 101; the first hole-transport layer 113 a over thehole-injection layer 111; the second hole-transport layer 113 b over thehole-injection layer 111; the third hole-transport layer 113 c over thehole-injection layer 111; the first light-emitting layer 115 a over thefirst hole-transport layer 113 a; the second light-emitting layer 115 bover the second hole-transport layer 113 b; the third light-emittinglayer 115 c over the third hole-transport layer 113 c; theelectron-transport layer 117 over the first light-emitting layer 115 a,the second light-emitting layer 115 b, and the third light-emittinglayer 115 c; the electron-injection layer 119 over theelectron-transport layer 117; and the cathode 103 over theelectron-injection layer 119.

In this manner, layers other than the light-emitting layer 115 and theelectron-transport layer 117, such as a layer including a hole-injectionor electron-injection substance, a layer including a hole-transport orelectron-transport substance, a layer including a bipolar substance (amaterial having a high electron-transport property or a highhole-transport property), or the like may be formed between the pair ofelectrodes as necessary. However, such layers are not indispensable.

In the light-emitting element in FIG. 1A, the first hole-transport layer113 a, the second hole-transport layer 113 b, and the thirdhole-transport layer 113 c are provided on the respective light-emittinglayers (i.e., the first light-emitting layer 115 a, the secondlight-emitting layer 115 b, and the third light-emitting layer 115 c).However, one embodiment of the present invention is not limited thereto,and one hole-transport layer may be formed to be shared by thelight-emitting layers. Further, in the light-emitting element in FIG.1A, the optical path of light emitted from each light-emitting layer canbe adjusted by adjusting the thickness of the first hole-transport layer113 a, the second hole-transport layer 113 b, or the thirdhole-transport layer 113 c.

In addition, in the light-emitting element in FIG. 1A, theelectron-transport layer 117, the electron-injection layer 119, and thecathode 103 are shared by the light-emitting layer 115 (the firstlight-emitting layer 115 a, the second light-emitting layer 115 b, andthe third light-emitting layer 115 c). When the electron-transport layer117, the electron-injection layer 119, and the cathode 103 are used tobe shared by the light-emitting layer 115 in such a manner, productivityat the time of forming the light-emitting element can be increased. Notethat at the time of forming the light-emitting element in FIG. 1A, thefirst hole-transport layer 113 a, the second hole-transport layer 113 b,and the third hole-transport layer 113 c are each formed over thehole-injection layer 111, and the first light-emitting layer 115 a, thesecond light-emitting layer 115 b, and the third light-emitting layer115 c are each formed over the first hole-transport layer 113 a, thesecond hole-transport layer 113 b, and the third hole-transport layer113 c, respectively, by a separate-deposition step. The hole-transportlayers and the light-emitting layers are sequentially formed, wherebythe number of times of separate deposition can be reduced. For example,the first hole-transport layer 113 a and the first light-emitting layer115 a are formed successively, the second hole-transport layer 113 b andthe second light-emitting layer 115 b are formed successively, and thethird hole-transport layer 113 c and the third light-emitting layer 115c are formed successively. Therefore, the light-emitting element in FIG.1A can be formed by three-time separate deposition.

The first electron-transport material 122 a and the secondelectron-transport material 132 a in the light-emitting element in FIG.1A have an extremely high electron-transport property. Therefore,light-emitting regions of the first light-emitting layer 115 a and thesecond light-emitting layer 115 b are formed in the vicinities of thefirst hole-transport layer 113 a and the second hole-transport layer 113b. Therefore, light emission from the first light-emitting layer 115 aand the second light-emitting layer 115 b are not influenced or hardlyinfluenced by the triplet excitation energy level of theelectron-transport layer 117 even through the triplet excitation energylevel of the electron-transport layer 117 is lower than those of thefirst electron-transport material 122 a and the secondelectron-transport material 132 a.

In other words, in the light-emitting element of one embodiment of thepresent invention, even in the case where the electron-transport layer117 is used to be shared by the first light-emitting layer 115 a, thesecond light-emitting layer 115 b, and the third light-emitting layer115 c, each light-emitting layer can have an optimized elementstructure, whereby a light-emitting element with high productivity andhigh emission efficiency can be obtained.

Next, a light-emitting element illustrated in FIG. 1B is described.

A light-emitting element illustrated in FIG. 1B includes thelight-emitting layer 115 between a pair of electrodes (the anode 101 andthe cathode 103). The light-emitting layer 115 includes the firstlight-emitting layer 115 a including the first phosphorescent material121 a and the first electron-transport material 122 a; the secondlight-emitting layer 115 b including the second phosphorescent material131 a and the second electron-transport material 132 a; and the thirdlight-emitting layer 115 c covering the first light-emitting layer 115 aand second light-emitting layer 115 b and including the fluorescentmaterial 141 a and the third electron-transport material 142 a.

The third light-emitting layer 115 c is provided on and in contact withthe first light-emitting layer 115 a and the second light-emitting layer115 b on the cathode 103 side.

The first light-emitting layer 115 a may further include the firsthole-transport material 123 a, in addition to the first phosphorescentmaterial 121 a and the first electron-transport material 122 a. Thesecond light-emitting layer 115 b may further include the secondhole-transport material 133 a, in addition to the second phosphorescentmaterial 131 a and the second electron-transport material 132 a.

In FIG. 1B, in addition to the light-emitting layer 115, thehole-injection layer 111, the first hole-transport layer 113 a, thesecond hole-transport layer 113 b, the third hole-transport layer 113 c,and the electron-injection layer 119 are provided between the pair ofelectrodes. However, these layers may be provided as needed.

Specifically, the light-emitting element illustrated in FIG. 1B includesthe anode 101 over the substrate 100; the hole-injection layer 111 overthe anode 101; the first hole-transport layer 113 a over thehole-injection layer 111; the second hole-transport layer 113 b over thehole-injection layer 111; the third hole-transport layer 113 c over thehole-injection layer 111; the first light-emitting layer 115 a over thefirst hole-transport layer 113 a; the second light-emitting layer 115 bover the second hole-transport layer 113 b; the third light-emittinglayer 115 c over the first hole-transport layer 113 a, the secondhole-transport layer 113 b, and the third hole-transport layer 113 c;the electron-injection layer 119 over the third light-emitting layer 115c; and the cathode 103 over the electron-injection layer 119.

In the light-emitting element in FIG. 1B, the third light-emitting layer115 c functions as a light-emitting layer and also functions as anelectron-transport layer of each of the first light-emitting layer 115 aand the second light-emitting layer 115 b.

Note that since the first electron-transport material 122 a and thesecond electron-transport material 132 a have a high electron-transportproperty, the fluorescent material 141 a included in the thirdlight-emitting layer 115 c does not contribute to light emission in thefirst light-emitting layer 115 a and the second light-emitting layer 115b. On the other hand, in the third light-emitting layer 115 c, lightemission can be obtained from the fluorescent material 141 a included inthe third light-emitting layer 115 c.

That is, since the third light-emitting layer 115 c has a function of anelectron-transport layer and a function of a light-emitting layer at thesame time in the light-emitting element of one embodiment of the presentinvention, the third light-emitting layer 115 c over the firstlight-emitting layer 115 a and the second light-emitting layer 115 b canbe used to be shared by the first light-emitting layer 115 a and thesecond light-emitting layer 115 b as an electron-transport layer, andthe third light-emitting layer 115 c over the third hole-transport layer113 c can be used as a light-emitting layer. Accordingly, alight-emitting element with high productivity and high emissionefficiency can be obtained. Note that at the time of forming thelight-emitting element in FIG. 1B, the first hole-transport layer 113 a,the second hole-transport layer 113 b, and the third hole-transportlayer 113 c are each formed over the hole-injection layer 111, and thefirst light-emitting layer 115 a and the second light-emitting layer 115b are each formed over the first hole-transport layer 113 a and thesecond hole-transport layer 113 b, respectively, and the thirdlight-emitting layer 115 c is formed over the first light-emitting layer115 a, the second light-emitting layer 115 b, and the thirdhole-transport layer 113 c, by a separate-deposition step. Thehole-transport layers and the light-emitting layers are sequentiallyformed, whereby the number of times of separate deposition can bereduced. For example, the first hole-transport layer 113 a and the firstlight-emitting layer 115 a are formed successively, the secondhole-transport layer 113 b and the second light-emitting layer 115 b areformed successively, and the then third hole-transport layer 113 c isformed. After that, the third light-emitting layer 115 c is formed overthe first light-emitting layer 115 a, the second light-emitting layer115 b, and the third hole-transport layer 113 c. Therefore, thelight-emitting element in FIG. 1B can be formed by three-time separatedeposition. A step for forming the electron-transport layer 117, whichis performed in the formation process of the light-emitting element inFIG. 1A, can be omitted in the formation process of the light-emittingelement in FIG. 1B.

Here, other component elements of the light-emitting elements in FIGS.1A and 1B are described below in detail.

<Substrate>

The substrate 100 is used as a support of the light-emitting element.For example, glass, quartz, plastic, or the like can be used for thesubstrate 100. Alternatively, a flexible substrate can be used. Aflexible substrate is a substrate that can be bent (is flexible);examples of the flexible substrate include a plastic substrate made of apolycarbonate, a polyarylate, or a polyethersulfone, and the like. Afilm (made of polypropylene, a polyester, poly(vinyl fluoride),poly(vinyl chloride), or the like), an inorganic film formed byevaporation, or the like can be used. Note that another material may beused as long as the material functions as a support medium in themanufacturing process of the light emitting element.

<Anode>

The anode 101 can be formed using one or more kinds of conductive metalsand alloys, conductive compounds, and the like. In particular, it ispreferable to use a material with a high work function (4.0 eV or more).Examples include indium tin oxide (ITO), indium tin oxide containingsilicon or silicon oxide, indium zinc oxide, indium oxide containingtungsten oxide and zinc oxide, graphene, gold, platinum, nickel,tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, and anitride of a metal material (e.g., titanium nitride). Alternatively, theanode 101 may be formed as follows: silver, copper, aluminum, titanium,or the like is formed to have a nanowire shape (or a thin-stripe shape),and then a conductive substance (a conductive organic material,graphene, or the like) is formed thereover by a coating method, aprinting method, or the like.

<Cathode>

The cathode 103 can be formed using one or more kinds of conductivemetals and alloys, conductive compounds, and the like. In particular, itis preferable to use a material with a low work function (3.8 eV orless). Examples include aluminum, silver, an element belonging to Group1 or 2 of the periodic table (e.g., an alkali metal such as lithium orcesium, an alkaline earth metal such as calcium or strontium, ormagnesium), an alloy containing any of these elements (e.g., Mg—Ag orAl—Li), a rare earth metal such as europium or ytterbium, and an alloycontaining any of these rare earth metals.

<Hole-injection layer and hole-transport layer>

As the substance having a high hole-transport property used for thehole-injection layer 111, the first hole-transport layer 113 a, thesecond hole-transport layer 113 b, and the third hole-transport layer113 c, the following can be given, for example: aromatic amine compoundssuch as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation:NPB or a-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB);3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1);3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2);3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1); and the like. Alternatively, the followingcarbazole derivative can be used: 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene(abbreviation: TCPB), and9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-Carbazole (abbreviation: CzPA).These materials described here mainly are substances having a holemobility of greater than or equal to 10⁻⁶ cm²/(Vs). However, besides theabove materials, others may be used as long as the material has a higherhole-transport property than an electron-transport property.

Furthermore, for the hole-injection layer 111, the first hole-transportlayer 113 a, the second hole-transport layer 113 b, and the thirdhole-transport layer 113 c, a high molecular compound such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD) can be used.

In addition, a transition metal oxide can be given as an example of anacceptor substance which can be used for the hole-injection layer 111,the first hole-transport layer 113 a, the second hole-transport layer113 b, and the third hole-transport layer 113 c. As the transition metaloxide, oxides of metals belonging to Groups 4 to 8 of the periodic tableare preferable. Specifically, molybdenum oxide is particularlypreferable.

<First Light-Emitting Layer>

The first light-emitting layer 115 a includes the first phosphorescentmaterial 121 a (guest material), the first electron-transport material122 a (host material), and the first hole-transport material 123 a(assist material). Further, it is preferable that the firstlight-emitting layer 115 a emit red light.

Note that the T1 level of the host material (or the assist material) ispreferably higher than the T1 level of the guest material. This isbecause, when the T1 level of the host material is lower than that ofthe guest material, the triplet excitation energy of the guest material,which is to contribute to light emission, is quenched by the hostmaterial and accordingly the emission efficiency is decreased.

An exciplex can be formed with the combination of the firstphosphorescent material 121 a (guest material), the firstelectron-transport material 122 a (host material), and the firsthole-transport material 123 a (assist material). It is preferable thatthe emission spectrum of the exciplex overlap the absorption spectrum ofthe first phosphorescent material 121 a (guest material) and that a peakof the emission spectrum of the exciplex have a longer wavelength than apeak of the absorption spectrum of the first phosphorescent material 121a (guest material).

Here, for improvement in efficiency of energy transfer from a hostmaterial to a guest material, Forster mechanism (dipole-dipoleinteraction) and Dexter mechanism (electron exchange interaction), whichare known as mechanisms of energy transfer between molecules, areconsidered. According to the mechanisms, it is preferable that anemission spectrum of a host material (fluorescence spectrum in energytransfer from a singlet excited state, phosphorescence spectrum inenergy transfer from a triplet excited state) largely overlap with anabsorption spectrum of a guest material (specifically, spectrum in anabsorption band on the longest wavelength (lowest energy) side).

However, in general, it is difficult to obtain an overlap between afluorescence spectrum of a host material and an absorption spectrum inan absorption band on the longest wavelength (lowest energy) side of aguest material. The reason for this is as follows: if the fluorescencespectrum of the host material overlaps with the absorption spectrum inthe absorption band on the longest wavelength (lowest energy) side ofthe guest material, since a phosphorescence spectrum of the hostmaterial is located on a longer wavelength (lower energy) side than thefluorescence spectrum, the T1 level of the host material becomes lowerthan the T1 level of the phosphorescent compound and the above-describedproblem of quenching occurs; yet, when the host material is designed insuch a manner that the T1 level of the host material is higher than theT1 level of the phosphorescent compound to avoid the problem ofquenching, the fluorescence spectrum of the host material is shifted tothe shorter wavelength (higher energy) side, and thus the fluorescencespectrum does not have any overlap with the absorption spectrum in theabsorption band on the longest wavelength (lowest energy) side of theguest material. For that reason, in general, it is difficult to obtainan overlap between a fluorescence spectrum of a host material and anabsorption spectrum in an absorption band on the longest wavelength(lowest energy) side of a guest material so as to maximize energytransfer from a singlet excited state of the host material.

Thus, it is preferable that the first light-emitting layer 115 aincluded in the light-emitting element of one embodiment of the presentinvention include the first phosphorescent material 121 a serving as aguest material (referred to as a first substance), the firstelectron-transport material 122 a serving as a host material (referredto as a second substance), and the first hole-transport material 123 a(referred to as a third substance), and the combination of the hostmaterial and the third substance form an exciplex. In that case, thehost material and the third substance form an exciplex at the time ofrecombination of carriers (electrons and holes) in the light-emittinglayer.

Thus, in the light-emitting layer, fluorescence spectra of the hostmaterial and the third substance are converted into an emission spectrumof the exciplex which is located on a longer wavelength side. Moreover,when the host material and the third substance are selected such thatthe emission spectrum of the exciplex has a large overlap with theabsorption spectrum of the guest material, energy transfer from asinglet excited state can be maximized. Note that also in the case of atriplet excited state, energy transfer from the exciplex, not the hostmaterial, is assumed to occur. In one embodiment of the presentinvention to which such a structure is applied, energy transferefficiency can be improved owing to energy transfer utilizing an overlapbetween an emission spectrum of an exciplex and an absorption spectrumof a phosphorescent compound; accordingly, a light-emitting element withhigh external quantum efficiency can be provided.

In the case where the first electron-transport material 122 a (hostmaterial) and the first hole-transport material 123 a (assist material)are used, carrier balance can be controlled by a mixing ratio thereof.Specifically, the mixing ratio of the first electron-transport material122 a to the first hole-transport material 123 a is preferably in therange of 1:9 to 9:1 (weight ratio).

Energy transfer (exciton diffusion) between exciplexes is not likely tooccur; therefore, the use of the exciplex in the above manner canprevent exciton diffusion to the electron-transport layer 117.

A phosphorescent material having an emission peak at 600 nm to 700 nmcan be given as an example of the first phosphorescent material 121 aused for the first light-emitting layer 115 a. Examples of thephosphorescent material include organometallic iridium complexes havingpyrimidine skeletons, such as(diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III)(abbreviation: [Ir(5mdppm)₂(dibm)]),bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: [Ir(5mdppm)₂(dpm)]),and bis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: [Ir(d1npm)₂(dpm)]);organometallic iridium complexes having pyrazine skeletons, such as(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: [Ir(tppr)₂(acac)]), bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: [Ir(tppr)₂(dpm)]), and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: [Ir(Fdpq)₂(acac)]); and organometallic iridium complexeshaving pyridine skeletons, such astris(1-phenylisoquinolinato-N,C^(2′))iridium(III) (abbreviation:[Ir(piq)₃]) andbis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: [Ir(piq)₂(acac)]). Among the materials given above, theorganometallic iridium complexes having pyrimidine skeletons havedistinctively high reliability and emission efficiency and are thusespecially preferable. Further, the organometallic iridium complexeshaving pyrazine skeletons can provide red light emission with favorablechromaticity.

As the first electron-transport material 122 a that can be used for thefirst light-emitting layer 115 a, a π-electron deficient heteroaromaticcompound such as a nitrogen-containing heteroaromatic compound ispreferable; for example, the following can be given: heterocycliccompounds (e.g., an oxadiazole derivative, an imidazole derivative, anda triazole derivative) having polyazole skeletons, such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), and2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); heterocyclic compounds (e.g., a pyrazinederivative, a pyrimidine derivative, a pyridazine derivative, aquinoxaline derivative, and a dibenzoquinoxaline derivative) havingdiazine skeletons, such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h] quinoxaline(abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine(abbreviation: 4,6mPnP2Pm), and4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II); and heterocyclic compounds (e.g., a pyridinederivative, a quinoline derivative, and a dibenzoquinoline derivative)having pyridine skeletons, such as3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 3,5DCzPPy)and 1,3,5-tri[3-(3-pyridyl)-phen-3-yl]benzene (abbreviation: TmPyPB).Among the above materials, heterocyclic compounds having diazineskeletons and heterocyclic compounds having pyridine skeletons have highreliability and are thus preferable. Specifically, heterocycliccompounds having diazine (pyrimidine or pyrazine) skeletons have a highelectron-transport property to contribute to a reduction in drivevoltage.

As the first hole-transport material 123 a which can be used for thefirst light-emitting layer 115 a, a π-electron rich heteroaromaticcompound (e.g., a carbazole derivative or an indole derivative) or anaromatic amine compound is preferable; for example, the following can begiven: compounds having aromatic amine skeletons such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-fluoren-2-amine(abbreviation: PCBAF), andN-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF); compounds having carbazole skeletons such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),3,6-bis(3,5-diphenylphenyl)-9-phenyl-carbazole (abbreviation: CzTP), and3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); compounds havingthiophene skeletons such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III), and4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV); and compounds having furan skeletons such as4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-II),or 4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II). Among the above materials, compoundshaving aromatic amine skeletons and compounds having carbazole skeletonsare preferable because these compounds are highly reliable and have highhole-transport properties to contribute to a reduction in drive voltage.

<Second Light-Emitting Layer>

The second light-emitting layer 115 b includes the second phosphorescentmaterial 131 a (guest material), the second electron-transport material132 a (host material), and the second hole-transport material 133 a(assist material). Further, it is preferable that the secondlight-emitting layer 115 b emit green light.

An exciplex can be formed with the combination of the secondphosphorescent material 131 a (guest material), the secondelectron-transport material 132 a (host material), and the secondhole-transport material 133 a (assist material). It is preferable thatthe emission spectrum of the exciplex overlap the absorption spectrum ofthe second phosphorescent material 131 a (guest material) and that apeak of the emission spectrum of the exciplex have a longer wavelengththan a peak of the absorption spectrum of the second phosphorescentmaterial 131 a (guest material). Note that as for the structure of theexciplex, a structure similar to that of the first light-emitting layer115 a can be applied to the second light-emitting layer 115 b.

In the case where the second electron-transport material 132 a (hostmaterial) and the second hole-transport material 133 a (assist material)are used, carrier balance can be controlled by a mixing ratio thereof.Specifically, the mixing ratio of the second electron-transport material132 a to the second hole-transport material 133 a is preferably in therange of 1:9 to 9:1 (weight ratio).

Energy transfer (exciton diffusion) between exciplexes is not likely tooccur; therefore, the use of the exciplex in the above manner canprevent exciton diffusion to the electron-transport layer 117.

A phosphorescent material having an emission peak at 520 nm to 600 nmcan be given as an example of the second phosphorescent material 131 aused for the second light-emitting layer 115 b. Examples of thephosphorescent material include organometallic iridium complexes havingpyrimidine skeletons, such astris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation:[Ir(mppm)₃]), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₃]),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(mppm)₂(acac)]),(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]),(acetylacetonato)bis[4-(2-norbornyl)-6-phenylpyrimidinato]iridium(III)(endo- and exo-mixture) (abbreviation: [Ir(nbppm)₂(acac)]),(acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: [Ir(mpmppm)₂(acac)]), and(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: [Ir(dppm)₂(acac)]); organometallic iridium complexeshaving pyrazine skeletons, such as(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: [Ir(mppr-Me)₂(acac)]) and(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: [Ir(mppr-iPr)₂(acac)]); and organometallic iridiumcomplexes having pyridine skeletons, such astris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation:[Ir(ppy)₃]), bis(2-phenylpyridinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: [Ir(ppy)₂(acac)]),bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation:[Ir(bzq)₂(acac)]), tris(benzo[h] quinolinato)iridium(III) (abbreviation:[Ir(bzq)₃]), tris(2-phenylquinolinato-N,C^(2′))iridium(III)(abbreviation: [Ir(pq)₃]), andbis(2-phenylquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: [Ir(pq)₂(acac)]).

Among the materials given above, the organometallic iridium complexeshaving pyrimidine skeletons have distinctively high reliability andemission efficiency and thus especially preferable.

A material similar to the material of the first electron-transportmaterial 122 a can be used as the second electron-transport material 132a that can be used for the second light-emitting layer 115 b.Alternatively, a material similar to the material of the firsthole-transport material 123 a can be used as the second hole-transportmaterial 133 a that can be used for the second light-emitting layer 115b.

<Third Light-Emitting Layer>

The third light-emitting layer 115 c includes the fluorescent material141 a (guest material) and the third electron-transport material 142 a(host material). Further, it is preferable that the third light-emittinglayer 115 c emit blue light.

Examples of the fluorescent material 141 a which can be used for thethird light-emitting layer 115 c includeN,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N′-diphenyl-pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn),N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine (abbreviation:1,6mMemFLPAPrn),N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra-tert-butylperylene(abbreviation: TBP), and4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA). Fluorescent compounds having pyrene skeletonsare particularly preferable because of their high hole-trappingproperties, high emission efficiency, and high reliability. In addition,condensed aromatic diamine compounds typified by pyrenediamine compoundssuch as 1,6FLPAPrn and 1,6mMemFLPAPrn are particularly preferablebecause of their high hole-trapping properties, high emissionefficiency, and high reliability.

As the third electron-transport material 142 a which can be used for thethird light-emitting layer 115 c, for example, an organic compoundhaving an anthracene skeleton is preferable. As the organic compoundhaving an anthracene skeleton, for example, an electron-transportcompound which easily accepts holes such as9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA),9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA), 3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviated to DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),and 2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation:t-BuDBA) can be preferably used. In the light-emitting element of oneembodiment of the present invention, the third electron-transportmaterial 142 a preferably has an anthracene skeleton to have ahole-trapping property in addition to an electron-transport property.

<Electron-Transport Layer>

The electron-transport layer 117 is a layer including a substance havinga high electron-transport property. The triplet excitation energy levelof the material forming the electron-transport layer 117 is lower thanthose of the first electron-transport material 122 a and the secondelectron-transport material 132 a that are used for the firstlight-emitting layer 115 a and the second light-emitting layer 115 b. Assuch a material, a material similar to the third electron-transportmaterial 142 a that can be used for the third light-emitting layer 115 ccan be used.

<Electron-Injection Layer>

The electron-injection layer 119 is a layer that includes a substancehaving a high electron-injection property. For the electron-injectionlayer 119, an alkali metal compound or an alkaline earth metal compound,such as lithium fluoride (LiF), cesium fluoride (CsF), calcium fluoride(CaF₂), or lithium oxide (LiO_(x)) can be used. Alternatively, a rareearth metal compound like erbium fluoride (ErF₃) can be used.

Alternatively, a composite material in which an organic compound and anelectron donor (donor) are mixed may be used for the electron-injectionlayer 119. The composite material is superior in an electron-injectionproperty and an electron-transport property, since electrons aregenerated in the organic compound by the electron donor. In this case,the organic compound here is preferably a material excellent intransporting the generated electrons, and as the electron donor, asubstance exhibiting an electron-donating property with respect to theorganic compound may be used. Specifically, an alkali metal, an alkalineearth metal, and a rare earth metal are preferable, and lithium, cesium,magnesium, calcium, erbium, ytterbium, and the like are given. Further,an alkali metal oxide or an alkaline earth metal oxide is preferable,and for example, lithium oxide, calcium oxide, barium oxide, and thelike can be given. Alternatively, Lewis base such as magnesium oxide canalso be used. An organic compound such as tetrathiafulvalene(abbreviation: TTF) can also be used.

Note that the hole-injection layer 111, the first hole-transport layer113 a, the second hole-transport layer 113 b, the third hole-transportlayer 113 c, the first light-emitting layer 115 a, the secondlight-emitting layer 115 b, the third light-emitting layer 115 c, theelectron-transport layer 117, and the electron-injection layer 119 eachcan be formed by an evaporation method (including a vacuum evaporationmethod), an inkjet method, a coating method, or the like.

Light emission obtained from the first light-emitting layer 115 a, thesecond light-emitting layer 115 b, and the third light-emitting layer115 c of the light-emitting element is extracted to the outside throughone or both of the anode 101 and the cathode 103. Thus, one or both ofthe anode 101 and the cathode 103 in this embodiment are an electrodehaving a light-transmitting property.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments and examples.

Embodiment 2

In this embodiment, modification examples of the light-emitting elementsof one embodiment of the present invention in FIGS. 1A and 1B aredescribed with reference to FIGS. 2A and 2B and FIGS. 3A and 3B. Notethat portions similar to those in the above embodiments and portionshaving functions similar to those in the above embodiments are given thesame reference numerals, and detailed description thereof is omitted.

A light-emitting element illustrated in FIG. 2A includes thelight-emitting layer 115 between a pair of electrodes (the anode 101 andthe cathode 103). The light-emitting layer 115 includes the firstlight-emitting layer 115 a including the first phosphorescent material121 a and the first electron-transport material 122 a; the secondlight-emitting layer 115 b including the second phosphorescent material131 a and the second electron-transport material 132 a; and the thirdlight-emitting layer 115 c including the fluorescent material 141 a andthe third electron-transport material 142 a.

The first light-emitting layer 115 a, the second light-emitting layer115 b, and the third light-emitting layer 115 c are each in contact withthe electron-transport layer 117 which is positioned on the cathode 103side.

The first light-emitting layer 115 a may further include the firsthole-transport material 123 a, in addition to the first phosphorescentmaterial 121 a and the first electron-transport material 122 a. Thesecond light-emitting layer 115 b may further include the secondhole-transport material 133 a, in addition to the second phosphorescentmaterial 131 a and the second electron-transport material 132 a.

In FIG. 2A, in addition to the light-emitting layer 115 and theelectron-transport layer 117, the hole-injection layer 111, the firsthole-transport layer 113 a, the second hole-transport layer 113 b, afourth hole-transport layer 113 d, and the electron-injection layer 119are provided between the pair of electrodes.

Specifically, the light-emitting element illustrated in FIG. 2A includesthe anode 101 over the substrate 100; the hole-injection layer 111 overthe anode 101; the fourth hole-transport layer 113 d over thehole-injection layer 111; the first hole-transport layer 113 a over thefourth hole-transport layer 113 d; the second hole-transport layer 113 bover the fourth hole-transport layer 113 d; the first light-emittinglayer 115 a over the first hole-transport layer 113 a; the secondlight-emitting layer 115 b over the second hole-transport layer 113 b;the third light-emitting layer 115 c over the fourth hole-transportlayer 113 d; the electron-transport layer 117 over the firstlight-emitting layer 115 a, the second light-emitting layer 115 b, andthe third light-emitting layer 115 c; the electron-injection layer 119over the electron-transport layer 117; and the cathode 103 over theelectron-injection layer 119.

Next, a light-emitting elements illustrated in FIG. 2B is describedbelow.

A light-emitting element illustrated in FIG. 2B includes thelight-emitting layer 115 between a pair of electrodes (the anode 101 andthe cathode 103). The light-emitting layer 115 includes the firstlight-emitting layer 115 a including the first phosphorescent material121 a and the first electron-transport material 122 a; the secondlight-emitting layer 115 b including the second phosphorescent material131 a and the second electron-transport material 132 a; and the thirdlight-emitting layer 115 c covering the first light-emitting layer 115 aand second light-emitting layer 115 b and including the fluorescentmaterial 141 a and the third electron-transport material 142 a.

The third light-emitting layer 115 c is in contact with the firstlight-emitting layer 115 a and the second light-emitting layer 115 b onthe cathode 103 side.

The first light-emitting layer 115 a may further include the firsthole-transport material 123 a, in addition to the first phosphorescentmaterial 121 a and the first electron-transport material 122 a. Thesecond light-emitting layer 115 b may further include the secondhole-transport material 133 a, in addition to the second phosphorescentmaterial 131 a and the second electron-transport material 132 a.

In FIG. 2B, in addition to the light-emitting layer 115, thehole-injection layer 111, the first hole-transport layer 113 a, thesecond hole-transport layer 113 b, the fourth hole-transport layer 113d, and the electron-injection layer 119 are provided between the pair ofelectrodes.

Specifically, the light-emitting element illustrated in FIG. 2B includesthe anode 101 over the substrate 100; the hole-injection layer 111 overthe anode 101; the fourth hole-transport layer 113 d over thehole-injection layer 111; the first hole-transport layer 113 a over thefourth hole-transport layer 113 d; the second hole-transport layer 113 bover the fourth hole-transport layer 113 d; the first light-emittinglayer 115 a over the first hole-transport layer 113 a; the secondlight-emitting layer 115 b over the second hole-transport layer 113 b;the third light-emitting layer 115 c over the first light-emitting layer115 a, the second light-emitting layer 115 b, and the fourthhole-transport layer 113 d; the electron-injection layer 119 over thethird light-emitting layer 115 c; and the cathode 103 over theelectron-injection layer 119.

The light-emitting elements in FIGS. 2A and 2B are different from thosein FIGS. 1A and 1B in that the fourth hole-transport layer 113 d isprovided over the hole-injection layer 111. Further, the thirdhole-transport layer 113 c is not provided in the third light-emittinglayer 115 c. That is, the third light-emitting layer 115 c is in contactwith the fourth hole-transport layer 113 d. A material similar to thatof the third hole-transport layer 113 c can be used for the fourthhole-transport layer 113 d.

The fourth hole-transport layer 113 d can be used to be shared by thefirst light-emitting layer 115 a, the second light-emitting layer 115 b,and the third light-emitting layer 115 c. Therefore, in thelight-emitting elements in FIGS. 2A and 2B, as well as an excellenteffect of the light-emitting elements in FIGS. 1A and 1B of oneembodiment of the present invention, productivity at the time of formingthe light-emitting element can be increased. Note that at the time offorming the light-emitting element in FIG. 2A, the first hole-transportlayer 113 a, the second hole-transport layer 113 b, the firstlight-emitting layer 115 a, the second light-emitting layer 115 b, andthe third light-emitting layer 115 c are each formed by aseparate-deposition step. The hole-transport layers and thelight-emitting layers are sequentially formed, whereby the number oftimes of separate deposition can be reduced. For example, the firsthole-transport layer 113 a and the first light-emitting layer 115 a areformed successively, the second hole-transport layer 113 b and thesecond light-emitting layer 115 b are formed successively, and then thethird light-emitting layer 115 c is formed. Therefore, thelight-emitting element in FIG. 2A can be formed by three-time separatedeposition. Further, at the time of forming the light-emitting elementin FIG. 2B, the first hole-transport layer 113 a, the secondhole-transport layer 113 b, the first light-emitting layer 115 a, andthe second light-emitting layer 115 b are each formed by aseparate-deposition step. Furthermore, successive formation of thehole-transport layers and the light-emitting layers can reduce thenumber of times for separate deposition. For example, the firsthole-transport layer 113 a and the first light-emitting layer 115 a aresuccessively formed, and the second hole-transport layer 113 b and thesecond light-emitting layer 115 b are successively formed. Thus, thelight-emitting element in FIG. 2B can be formed by two-time separatedeposition.

In the light-emitting elements in FIGS. 2A and 2B, the firstlight-emitting layer 115 a and the second light-emitting layer 115 b areseparately formed to be in contact with the first hole-transport layer113 a and the second hole-transport layer 113 b, respectively.Therefore, an optimal element structure can be achieved in eachlight-emitting layer, so that a light-emitting element with highemission efficiency can be achieved in each light-emitting layer.

In the light-emitting elements in FIGS. 2A and 2B, an optical path ineach light-emitting layer can be adjusted by adjusting the thicknessesof the first hole-transport layer 113 a, the second hole-transport layer113 b, and the fourth hole-transport layer 113 d.

Next, a light-emitting element illustrated in FIG. 3A is describedbelow.

A light-emitting element illustrated in FIG. 3A includes thelight-emitting layer 115 between a pair of electrodes (the anode 101 andthe cathode 103). The light-emitting layer 115 includes the firstlight-emitting layer 115 a including the first phosphorescent material121 a and the first electron-transport material 122 a; the secondlight-emitting layer 115 b including the second phosphorescent material131 a and the second electron-transport material 132 a; and the thirdlight-emitting layer 115 c including the fluorescent material 141 a andthe third electron-transport material 142 a.

Each of the first light-emitting layer 115 a, the second light-emittinglayer 115 b, and the third light-emitting layer 115 c is in contact withthe electron-transport layer 117 provided on the cathode 103 side.

The first light-emitting layer 115 a may further include the firsthole-transport material 123 a, in addition to the first phosphorescentmaterial 121 a and the first electron-transport material 122 a. Thesecond light-emitting layer 115 b may further include the secondhole-transport material 133 a, in addition to the second phosphorescentmaterial 131 a and the second electron-transport material 132 a.

In FIG. 3A, the hole-injection layer 111, the hole-transport layer 113,and the electron-injection layer 119 are provided between the pair ofelectrodes, in addition to the light-emitting layer 115.

Specifically, the light-emitting element in FIG. 3A includes the anode101 over the substrate 100; the hole-injection layer 111 over the anode101; the hole-transport layer 113 over the hole-injection layer 111; thefirst light-emitting layer 115 a over the hole-transport layer 113; thesecond light-emitting layer 115 b over the hole-transport layer 113; thethird light-emitting layer 115 c over the hole-transport layer 113; theelectron-injection layer 119 over the first light-emitting layer 115 a,the second light-emitting layer 115 b, and the third light-emittinglayer 115 c; and the cathode 103 over the electron-injection layer 119.

Next, a light-emitting elements illustrated in FIG. 3B is describedbelow.

A light-emitting element illustrated in FIG. 3B includes thelight-emitting layer 115 between a pair of electrodes (the anode 101 andthe cathode 103). The light-emitting layer 115 includes the firstlight-emitting layer 115 a including the first phosphorescent material121 a and the first electron-transport material 122 a; the secondlight-emitting layer 115 b including the second phosphorescent material131 a and the second electron-transport material 132 a; and the thirdlight-emitting layer 115 c covering the first light-emitting layer 115 aand second light-emitting layer 115 b and including the fluorescentmaterial 141 a and the third electron-transport material 142 a.

The third light-emitting layer 115 c is in contact with the firstlight-emitting layer 115 a and the second light-emitting layer 115 b onthe cathode 103 side.

The first light-emitting layer 115 a may further include the firsthole-transport material 123 a, in addition to the first phosphorescentmaterial 121 a and the first electron-transport material 122 a. Thesecond light-emitting layer 115 b may further include the secondhole-transport material 133 a, in addition to the second phosphorescentmaterial 131 a and the second electron-transport material 132 a.

In FIG. 3B, in addition to the light-emitting layer 115, thehole-injection layer 111, the hole-transport layer 113, and theelectron-injection layer 119 are provided between the pair ofelectrodes.

Specifically, the light-emitting element in FIG. 3B includes the anode101 over the substrate 100; the hole-injection layer 111 over the anode101; the hole-transport layer 113 over the hole-injection layer 111; thefirst light-emitting layer 115 a over the hole-transport layer 113; thesecond light-emitting layer 115 b over the hole-transport layer 113; thethird light-emitting layer 115 c over the first light-emitting layer 115a, the second light-emitting layer 115 b, and the hole-transport layer113; the electron-injection layer 119 over the third light-emittinglayer 115 c; and the cathode 103 over the electron-injection layer 119.

The light-emitting elements in FIGS. 3A and 3B are different from thosein FIGS. 1A and 1B in that the hole-transport layer 113 is formed overthe hole-injection layer 111. That is, the hole-transport layer 113 canbe used as a hole-transport layer which is shared by the firstlight-emitting layer 115 a, the second light-emitting layer 115 b, andthe third light-emitting layer 115 c. A material similar to that of thethird hole-transport layer 113 c can be used for the hole-transportlayer 113. Therefore, in the light-emitting elements in FIGS. 3A and 3B,as well as an excellent effect of the light-emitting elements in FIGS.1A and 1B of one embodiment of the present invention, productivity atthe time of forming the light-emitting element can be increased. Notethat in the case where the light-emitting element in FIG. 3A is formed,the first light-emitting layer 115 a, the second light-emitting layer115 b, and the third light-emitting layer 115 c are each formed by aseparate-deposition step; that is, the total number ofseparate-deposition steps is three. In the case where the light-emittingelement in FIG. 3B, the first light-emitting layer 115 a and the secondlight-emitting layer 115 b are each formed by a separate-depositionstep; that is, the total number of separate-deposition steps is two.

Note that in the element structures in FIGS. 3A and 3B, since thehole-transport layer 113 is used to be shared by the firstlight-emitting layer 115 a, the second light-emitting layer 115 b, andthe third light-emitting layer 115 c, one or two of the firstlight-emitting layer 115 a, the second light-emitting layer 115 b andthe third light-emitting layer 115 c might be reduced in their elementcharacteristics. Note that in the case where productivity takesprecedence of element characteristics, the structures in FIGS. 3A and 3Bmay be applied. The first electron-transport material 122 a, the secondelectron-transport material 132 a, and the third light-emitting layer115 c in the structures in FIGS. 3A and 3B have an extremely highelectron-transport property. Therefore, even in the case where ahole-transport layer is used to be shared by light-emitting layers, theelement characteristics on the electron-transport layer side are notreduced or hardly reduced; accordingly a light-emitting element in whichthe plurality of light-emitting layers are balanced can be obtained.

The light-emitting elements in FIGS. 3A and 3B each have the structurein which the anode 101 provided in the lower portion is used to beshared by the first light-emitting layer 115 a, the secondlight-emitting layer 115 b, and the third light-emitting layer 115 c;however, one embodiment of the present invention is not limited thereto.For example, it is possible to employ a structure in which anodes 101having different thicknesses are respectively provided below the firstlight-emitting layer 115 a, the second light-emitting layer 115 b, andthe third light-emitting layer 115 c. For example, in the structureincluding anodes 101 having different thicknesses, the anode 101 of thefirst light-emitting layer 115 a may have the largest thickness, that ofthe second light-emitting layer 115 b may have the second largestthickness, and that of the third light-emitting layer 115 c may have thesmallest thickness.

Since the hole-transport layer 113 is used to be shared by thelight-emitting layers in the light-emitting elements in FIGS. 3A and 3B,the structure in which the optical path is adjusted by the thickness ofthe anode 101 is one of structures that are effective in improving theelement characteristics of each light-emitting layer.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments and examples.

Embodiment 3

In this embodiment, a light-emitting device manufactured using thelight-emitting element of one embodiment of the present invention isdescribed with reference to FIGS. 4A and 4B.

FIGS. 4A and 4B are cross-sectional views of light-emitting devices 250and 260 in which first to third light-emitting layers are providedbetween an anode and a cathode.

First, the light-emitting device 250 illustrated in FIG. 4A is describedbelow.

The light-emitting device 250 is what is called a bottom-emissionlight-emitting device in which light can be extracted from a substrate200 side (a side indicated by arrows in FIG. 4A).

The light-emitting device 250 includes anodes 201 a, 201 b, and 201 cthat have shapes of separated islands, over the substrate 200. Thematerial of the substrate 100 described in Embodiment 1 can be used forthe substrate 200. The material of the anode 101 described in Embodiment1 can be used for the anodes 201 a, 201 b, and 201 c. The anodes 201 a,201 b, and 201 c may have different thicknesses depending on an emissioncolor of the element. Further, since the light-emitting device 250 is abottom-emission light-emitting device, the anodes 201 a, 201 b, and 201c are preferably formed using a material which transmits visible light(e.g., ITO).

The light-emitting device 250 includes partitions 251 a, 251 b, 251 c,and 251 d. The partition 251 a covers one end portion of the anode 201a. The partition 251 b covers the other end portion of the anode 201 aand one end portion of the anode 201 b. The partition 251 c covers theother end portion of the anode 201 b and one end portion of the anode201 c. The partition 251 d covers the other end portion of the anode 201c. The partitions 251 a, 251 b, 251 c, and 251 d can be formed using anorganic resin or an inorganic resin. As the organic resin, for example,a polyimide resin, a polyamide resin, an acrylic resin, a siloxaneresin, an epoxy resin, a phenol resin, or the like can be used. As theinorganic insulating material, silicon oxide, silicon oxynitride, or thelike can be used. In particular, a photosensitive resin is preferablyused for easy formation of the partitions 251 a, 251 b, 251 c, and 251d.

The light-emitting device 250 includes a hole-injection layer 211 overthe anodes 201 a, 201 b, and 201 c and the partitions 251 a, 251 b, 251c, and 251 d. The material of the hole-injection layer 111 described inEmbodiment 1 can be used for the hole-injection layer 211.

The light-emitting device 250 includes a first hole-transport layer 213a, a second hole-transport layer 213 b, and a third hole-transport layer213 c which are separately provided over the hole-injection layer 211 tohave island shapes. A first light-emitting layer 215 a, a secondlight-emitting layer 215 b, and a third light-emitting layer 215 c areprovided over the first hole-transport layer 213 a, the secondhole-transport layer 213 b, and the third hole-transport layer 213 c,respectively. The first hole-transport layer 213 a, the secondhole-transport layer 213 b, the third hole-transport layer 213 c, thefirst light-emitting layer 215 a, the second light-emitting layer 215 b,and the third light-emitting layer 215 c can be respectively formedusing the material of the first hole-transport layer 113 a, that of thesecond hole-transport layer 113 b, that of the third hole-transportlayer 113 c, that of the first light-emitting layer 115 a, that of thesecond light-emitting layer 115 b, and that of the third light-emittinglayer 115 c, which are described in Embodiment 1.

In a manner similar to the first light-emitting layer 115 a in FIG. 1A,the first light-emitting layer 215 a includes a first phosphorescentmaterial, a first electron-transport material, and a firsthole-transport material. In a manner similar to the secondlight-emitting layer 115 b in FIG. 1A, the second light-emitting layer215 b includes a second phosphorescent material, a secondelectron-transport material, and a second hole-transport material. In amanner similar to the third light-emitting layer 115 c in FIG. 1A, thethird light-emitting layer 215 c includes a fluorescent material and athird electron-transport material. Note that to avoid complexity of thedrawing, the first phosphorescent material, the first electron-transportmaterial, the first hole-transport material, the second phosphorescentmaterial, the second electron-transport material, the secondhole-transport material, the fluorescent material, and the thirdelectron-transport material are not illustrated in FIG. 4A.

The light-emitting device 250 includes an electron-transport layer 217over the first light-emitting layer 215 a, the second light-emittinglayer 215 b, and the third light-emitting layer 215 c. Further, anelectron-injection layer 219 is provided over the electron-transportlayer 217. Furthermore, a cathode 203 is provided over theelectron-injection layer 219. The material of the electron-transportlayer 117 described in Embodiment 1 can be used for theelectron-transport layer 217. The material of the electron-injectionlayer 119 described in Embodiment 1 can be used for theelectron-injection layer 219. The material of the cathode 103 describedin Embodiment 1 can be used for the cathode 203. Since thelight-emitting device 250 is a bottom-emission light-emitting device,the cathode 203 is preferably formed using a reflective material (e.g.,aluminum).

Although FIG. 4A shows the example in which the anode is provided in thelower portion and the cathode is provided in the upper portion, oneembodiment of the present invention is not limited thereto. For example,a structure in which the anode is provided in the upper portion and thecathode is provided in the lower portion may be employed. In this case,the stacking order of the hole-injection layer, the hole-transportlayer, the light-emitting layer, the electron-injection layer, and theelectron-transport layer, which are provided between the anode and thecathode, may be changed.

The first light-emitting layer 215 a, the second light-emitting layer215 b, and the third light-emitting layer 215 c of the light-emittingdevice 250 are in contact with the electron-transport layer 217. Thetriplet excitation energy level of the material forming theelectron-transport layer 217 is lower than that of theelectron-transport material included in the first light-emitting layer215 a and that of the second electron-transport material included in thelight-emitting layer 215 b. In this manner, even when theelectron-transport layer is used to be shared by the light-emittinglayers included in the light-emitting device 250, an optimal elementstructure is obtained and thus low driving voltage, high currentefficiency, or long lifetime is achieved. Therefore, the light-emittingdevice 250 with low power consumption or long lifetime can be obtained.Further, since the electron-transport layer is used to be shared, thelight-emitting device 250 with high productivity can be obtained.

Next, the light-emitting device 260 in FIG. 4B is described below.

The light-emitting device 260 is a modification example of thelight-emitting device 250 and is what is called a top-emissionlight-emitting device in which light can be extracted from a sideindicated by arrows in FIG. 4B.

The light-emitting device 260 includes reflective electrodes 253 a, 253b, and 253 c that have shapes of separated islands, over the substrate200. Further, the light-emitting device 260 includes the anodes 201 a,201 b, and 201 c that have shapes of separated islands, over thereflective electrodes 253 a, 253 b, and 253 c. Since the light-emittingdevice 260 is a top-emission light-emitting device, the reflectiveelectrodes 253 a, 253 b, and 253 c are preferably formed using areflective material (e.g., aluminum or silver).

The light-emitting device 260 includes the partitions 251 a, 251 b, 251c, and 251 d. The partition 251 a covers one end portion of thereflective electrode 253 a and one end portion of the anode 201 a. Thepartition 251 b covers the other end portion of the reflective electrode253 a, the other end portion of the anode 201 a, one end portion of thereflective electrode 253 b, and one end portion of the anode 201 b. Thepartition 251 c covers the other end portion of the reflective electrode253 b, the other end portion of the anode 201 b, one end portion of thereflective electrode 253 c, and one end portion of the anode 201 c. Thepartition 251 d covers the other end portion of the reflective electrode253 c and the other end portion of the anode 201 c.

The light-emitting device 260 includes the hole-injection layer 211 overthe anodes 201 a, 201 b, and 201 c and the partitions 251 a, 251 b, 251c, and 251 d.

The light-emitting device 260 includes the first hole-transport layer213 a, the second hole-transport layer 213 b, and the thirdhole-transport layer 213 c which are separately provided over thehole-injection layer 211 to have island shapes. The first light-emittinglayer 215 a, the second light-emitting layer 215 b, and the thirdlight-emitting layer 215 c are provided over the first hole-transportlayer 213 a, the second hole-transport layer 213 b, and the thirdhole-transport layer 213 c, respectively.

Note that in a manner similar to the first light-emitting layer 115 a inFIG. 1A, the first light-emitting layer 215 a includes a firstphosphorescent material, a first electron-transport material, and afirst hole-transport material. In a manner similar to the secondlight-emitting layer 115 b in FIG. 1A, the second light-emitting layer215 b includes a second phosphorescent material, a secondelectron-transport material, and a second hole-transport material. In amanner similar to the third light-emitting layer 115 c in FIG. 1A, thethird light-emitting layer 215 c includes a fluorescent material and athird electron-transport material. Note that to avoid complexity of thedrawing, the first phosphorescent material, the first electron-transportmaterial, the first hole-transport material, the second phosphorescentmaterial, the second electron-transport material, the secondhole-transport material, the fluorescent material, and the thirdelectron-transport material are not illustrated in FIG. 4B.

The light-emitting device 260 includes the electron-transport layer 217over the first light-emitting layer 215 a, the second light-emittinglayer 215 b, and the third light-emitting layer 215 c. Further, theelectron-injection layer 219 is provided over the electron-transportlayer 217. Furthermore, a semi-transmissive and semi-reflectiveelectrode 253 serving as a cathode is provided over theelectron-injection layer 219. For example, the semi-transmissive andsemi-reflective electrode 253 can be formed by stacking a thin metalfilm (preferably with a thickness of 20 nm or less, further preferably10 nm or less) and a conductive metal oxide film. The thin metal filmcan be formed using a single layer or a stacked layer using silver,magnesium, an alloy containing such a metal material, or the like.Indium oxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), ITO, an indiumoxide-zinc oxide (In₂O₃—ZnO), or any of these metal oxide materialscontaining silicon oxide can be used as the conductive metal oxide.

Since the light-emitting device 260 is a top-emission light-emittingdevice, the light strength of a certain wavelength can be increased byproviding a micro optical resonator (a microcavity) utilizing a resonanteffect of light between the semi-transmissive and semi-reflectiveelectrode 253 and the reflective electrodes 253 a, 253 b, and 253 c. Thefunction as a microcavity can be adjusted by a material provided betweenthe reflective electrodes 253 a, 253 b, and 253 c and thesemi-transmissive and semi-reflective electrode 253, a light pathlength, and the like. For example, the light strength of a certainwavelength emitted from each light-emitting layer may be increased byadjusting the thicknesses of the anodes 201 a, 201 b, and 201 c, thefirst hole-transport layer 213 a, the second hole-transport layer 213 b,and the third hole-transport layer 213 c. Note that the example of thelight-emitting device 260 in which the optical path length is adjustedby the thicknesses of the first hole-transport layer 213 a, the secondhole-transport layer 213 b, and the third hole-transport layer 213 c isdescribed here.

Although FIG. 4B shows the example in which the anode is provided in thelower portion and the cathode is provided in the upper portion, oneembodiment of the present invention is not limited thereto. For example,a structure in which the anode is provided in the upper portion and thecathode is provided in the lower portion may be employed. In this case,the stacking order of the hole-injection layer, the hole-transportlayer, the light-emitting layer, the electron-injection layer, and theelectron-transport layer, which are provided between the anode and thecathode, may be changed.

The first light-emitting layer 215 a, the second light-emitting layer215 b, and the third light-emitting layer 215 c of the light-emittingdevice 260 are in contact with the electron-transport layer 217. Thetriplet excitation energy level of the material forming theelectron-transport layer 217 is lower than that of theelectron-transport material included in the first light-emitting layer215 a and that of the second electron-transport material included in thesecond light-emitting layer 215 b. In this manner, even when theelectron-transport layer is used to be shared by the light-emittinglayers included in the light-emitting device 260, an optimal elementstructure is obtained and thus low driving voltage, high currentefficiency, or long lifetime is achieved. Therefore, the light-emittingdevice 260 with low power consumption or long lifetime can be obtained.Further, since the electron-transport layer is used to be shared, thelight-emitting device 260 with high productivity can be obtained.

Although FIGS. 4A and 4B show the examples of the light-emitting devices250 and 260 in which only the light-emitting element is formed over thesubstrate 200, one embodiment of the present invention is not limitedthereto. For example, it is preferable to employ a structure in which atransistor (e.g., a TFT) is separately formed over the substrate 200 andthe transistor is electrically connection to the anodes 201 a, 201 b,and 201 c or the reflective electrodes 253 a, 253 b, and 253 c.

Here, a method for manufacturing the light-emitting device 250illustrated in FIG. 4A is described below.

First, a conductive film is formed over the substrate 200 and processedinto a desired shape to form the anodes 201 a, 201 b, and 201 c. Next,the partitions 251 a, 251 b, 251 c, and 251 d are formed over thesubstrate 200 and the anodes 201 a, 201 b, and 201 c. Note that theanodes 201 a, 201 b, and 201 c and the partitions 251 a, 251 b, 251 c,and 251 d are preferably formed using a manufacturing process of thetransistor.

The structure of the transistor is not limited: a top-gate transistormay be used, or a bottom-gate transistor such as an inverted staggeredtransistor may be used. An n-channel transistor may be used and ap-channel transistor may also be used. In addition, there is noparticular limitation on a material used for the transistor. Forexample, a transistor in which silicon or an oxide semiconductor such asan In—Ga—Zn-based metal oxide is used in a channel formation region canbe employed.

Next, the hole-injection layer 211 is formed over the anodes 201 a, 201b, and 201 c, and the partitions 251 a, 251 b, 251 c, and 251 d. Theanodes 201 a, 201 b, and 201 c can be formed by an evaporation method(including a vacuum vapor deposition), a sputtering method, a coatingmethod, or an inkjet method. The hole-injection layer 211 can be formedby an evaporation method (including a vacuum evaporation method), atransfer method, a printing method, an inkjet method, a coating method,or the like.

Then, the first hole-transport layer 213 a is formed in contact with thehole-injection layer 211 to overlap the anode 201 a. The firsthole-transport layer 213 a can be formed by an evaporation method(including a vacuum evaporation method), a transfer method, a printingmethod, an inkjet method, a coating method, or the like. In thisembodiment, the first hole-transport layer 213 a is formed over adesired region by an evaporation method using a deposition mask (alsoreferred to as a metal mask, fine metal mask, or a shadow mask).

After that, the first light-emitting layer 215 a is formed over thefirst hole-transport layer 213 a. The first light-emitting layer 215 acan be formed by an evaporation method (including a vacuum evaporationmethod), a transfer method, a printing method, an inkjet method, acoating method, or the like. In this embodiment, the firstlight-emitting layer 215 a is formed over a desired region by anevaporation method using a deposition mask (also referred to as a metalmask, fine metal mask, or a shadow mask). Note that the firsthole-transport layer 213 a and the first light-emitting layer 215 a arepreferably formed successively using the same deposition mask.

Next, the second hole-transport layer 213 b is formed in contact withthe hole-injection layer 211 to overlap the anode 201 b. The secondhole-transport layer 213 b can be formed in the same way as that of thefirst hole-transport layer 213 a.

After that, the second light-emitting layer 215 b is formed over thesecond hole-transport layer 213 b. The second light-emitting layer 215 bcan be formed in the same way as that of the first light-emitting layer215 a. Note that the second hole-transport layer 213 b and the secondlight-emitting layer 215 b are preferably formed successively using thesame deposition mask.

Next, the third hole-transport layer 213 c is formed in contact with thehole-injection layer 211 to overlap the anode 201 c. The thirdhole-transport layer 213 c can be formed in the same way as that of thefirst hole-transport layer 213 a.

After that, the third light-emitting layer 215 c is formed over thethird hole-transport layer 213 c. The third light-emitting layer 215 ccan be formed in the same way as that of the first light-emitting layer215 a. Note that the third hole-transport layer 213 c and the thirdlight-emitting layer 215 c are preferably formed successively using thesame deposition mask.

Next, the electron-transport layer 217 is formed over the hole-injectionlayer 211, the first light-emitting layer 215 a, the secondlight-emitting layer 215 b, and the third light-emitting layer 215 c,and then the electron-injection layer 219 is formed over theelectron-transport layer 217. The electron-transport layer 217 and theelectron-injection layer 219 are each can be formed by an evaporationmethod (including a vacuum evaporation method), a transfer method, aprinting method, an inkjet method, a coating method, or the like.

Then, the cathode 203 is formed over the electron-injection layer 219.The cathode 203 can be formed by an evaporation method (including avacuum vapor deposition), a sputtering method, a coating method, or aninkjet method.

In this manner, the light-emitting device 250 in FIG. 4A can bemanufactured.

A step of forming the reflective electrodes 253 a, 253 b, and 253 cbelow the anodes 201 a, 201 b, and 201 c, and a step of forming thesemi-transmissive and semi-reflective electrode 253 instead of thecathode 203 are added to the manufacturing process of the light-emittingdevice 250, whereby the light-emitting device 260 in FIG. 4B can beformed.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments and examples.

Embodiment 4

In this embodiment, examples of a variety of electronic devices andlighting devices that are each completed by use of a light-emittingelement or light-emitting device of one embodiment of the presentinvention are described with reference to FIGS. 5A to 5E.

Examples of electronic devices are a television device (also referred toas a television or a television receiver), a monitor of a computer orthe like, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game console, aportable information terminal, an audio reproducing device, a large-sizegame machine such as a pachinko machine, and the like.

An electronic device or a lighting device that has a light-emittingportion with a curved surface can be obtained with the use of thelight-emitting element of one embodiment of the present invention, whichis manufactured over a substrate having flexibility.

In addition, an electronic device or a lighting device that has asee-through light-emitting portion can be obtained with the use of thelight-emitting element of one embodiment of the present invention, inwhich a pair of electrodes are formed using a material having a propertyof transmitting visible light.

Further, a light-emitting device to which one embodiment of the presentinvention is applied can also be applied to lighting for motor vehicles,examples of which are lighting for a dashboard, a windshield, a ceiling,and the like.

FIG. 5A illustrates an example of a television device. In a televisiondevice 7100, a display portion 7103 is incorporated in a housing 7101.Images can be displayed by the display portion 7103, and thelight-emitting device can be used for the display portion 7103. Inaddition, here, the housing 7101 is supported by a stand 7105.

The television device 7100 can be operated by an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Furthermore, the remote controller 7110 may be provided witha display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 5B illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured using the light-emitting device for thedisplay portion 7203.

FIG. 5C illustrates a portable game machine having two housings, ahousing 7301 and a housing 7302, which are connected with a jointportion 7303 so that the portable game machine can be opened or folded.A display portion 7304 is incorporated in the housing 7301 and a displayportion 7305 is incorporated in the housing 7302. In addition, theportable game machine illustrated in FIG. 5C includes a speaker portion7306, a recording medium insertion portion 7307, an LED lamp 7308, inputmeans (an operation key 7309, a connection terminal 7310, a sensor 7311(a sensor having a function of measuring force, displacement, position,speed, acceleration, angular velocity, rotational frequency, distance,light, liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), anda microphone 7312), and the like. It is needless to say that thestructure of the portable game machine is not limited to the above aslong as a light-emitting device is used for at least either the displayportion 7304 or the display portion 7305, or both, and may include otheraccessories as appropriate. The portable game machine illustrated inFIG. 5C has a function of reading out a program or data stored in astorage medium to display it on the display portion, and a function ofsharing information with another portable game machine by wirelesscommunication. The portable game machine illustrated in FIG. 5C can havea variety of functions without limitation to the above.

FIG. 5D illustrates an example of a mobile phone. A mobile phone 7400 isprovided with a display portion 7402 incorporated in a housing 7401, anoperation button 7403, an external connection port 7404, a speaker 7405,a microphone 7406, and the like. Note that the mobile phone 7400 ismanufactured using a light-emitting device for the display portion 7402.

When the display portion 7402 of the mobile phone 7400 illustrated inFIG. 5D is touched with a finger or the like, data can be input into themobile phone 7400. Further, operations such as making a call andcreating an e-mail can be performed by touching the display portion 7402with a finger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting data such as characters. Thethird mode is a display-and-input mode in which two modes of the displaymode and the input mode are combined.

For example, in the case of making a call or creating e-mail, acharacter input mode mainly for inputting characters is selected for thedisplay portion 7402 so that characters displayed on the screen can beinput. In this case, it is preferable to display a keyboard or numberbuttons on almost the entire screen of the display portion 7402.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone 7400, display on the screen of the display portion 7402 canbe automatically changed by determining the orientation of the mobilephone 7400 (whether the mobile phone is placed horizontally orvertically for a landscape mode or a portrait mode).

The screen modes are changed by touch on the display portion 7402 oroperation with the operation button 7403 of the housing 7401. The screenmodes can be switched depending on the kind of images displayed on thedisplay portion 7402. For example, when a signal of an image displayedon the display portion is a signal of moving image data, the screen modeis switched to the display mode. When the signal is a signal of textdata, the screen mode is switched to the input mode.

Moreover, in the input mode, if a signal detected by an optical sensorin the display portion 7402 is detected and the input by touch on thedisplay portion 7402 is not performed for a certain period, the screenmode may be controlled so as to be changed from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. Further, when a backlight or asensing light source which emits near-infrared light is provided in thedisplay portion, an image of a finger vein, a palm vein, or the like canbe taken.

FIG. 5E illustrates a desk lighting device including a lighting portion7501, a shade 7502, an adjustable arm 7503, a support 7504, a base 7505,and a power switch 7506. The desk lighting device is manufactured usingthe light-emitting device for the lighting portion 7501. Note that alighting device includes a ceiling light, a wall light, and the like inits category.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments and examples.

Example 1

In this embodiment, the triplet excitation energy levels (T1 levels) of9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA),which can be used for an electron-transport layer of a light-emittingelement of one embodiment of the present invention, and2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II), which can be used as a host material (afirst electron-transport material and a second electron-transportmaterial) of a phosphorescent element, were measured. Chemical formulaeof materials used in this example are shown below.

Note that the T1 levels were obtained by measurement of emission ofphosphorescence from the substances. In the measurement, each substancewas irradiated with excitation light with a wavelength of 325 nm and themeasurement temperature was 10 K. Further, 2mDBTBPDBq-II was subjectedto time-resolved measurement using mechanical choppers. It is difficultto apply time-resolved measurement to CzPA; therefore, Ir(ppy)₃ wasadded as a sensitizer and CzPA was measured without time resolving. Asthe measurement condition, the weight ratio of CzPA to Ir(ppy)₃ was 3:1.Note that in measuring a triplet excitation energy level, calculationfrom an absorption wavelength is more accurate than calculation from anemission wavelength. However, here, absorption of the T1 level wasextremely low and measuring it is difficult; thus, the T1 level wasmeasured by measuring an emission wavelength. For that reason, a fewerrors may be included in the measured values. Table 1 shows themeasurement results.

TABLE 1 Substance T1 level CzPA 1.72 eV 2mDBTBPDBq-II 2.41 eV

Table 1 shows that the triplet excitation energy level of CzPA which canbe used for an electron-transport layer is lower by 0.69 eV than that of2mDBTBPDBq-II which can be used as a host material (a firstelectron-transport material and a second electron-transport material) ofa phosphorescent element.

Example 2

In this example, light-emitting elements (light-emitting elements 1, 3,and 5) of one embodiment of the present invention and light-emittingelements for comparison (comparative light-emitting elements 2, 4, and6) are described with reference to FIG. 6A. Chemical formulae ofmaterials used in this example are shown below.

Methods for manufacturing the light-emitting elements (thelight-emitting elements 1, 3, and 5) of one embodiment of the presentinvention and the comparative light-emitting elements (the comparativelight-emitting elements 2, 4, and 6), which are used in this example,are described below.

Note that the light-emitting element 1 and the comparativelight-emitting element 2 are red-light-emitting elements, thelight-emitting element 3 and the comparative light-emitting element 4are green-light-emitting elements, and the light-emitting element 5 andthe comparative light-emitting element 6 are blue-light-emittingelements.

(Light-Emitting Element 1)

First, an indium oxide-tin oxide compound containing silicon or siliconoxide (ITO-SiO₂, hereinafter abbreviated to ITSO) was deposited over asubstrate 1100 by a sputtering method, whereby an anode 1101 was formed.Note that the composition ratio of In₂O₃ to SnO₂ and SiO₂ in the targetused was 85:10:5 [wt %]. The thickness of the anode 1101 was 110 nm andthe electrode area was 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, the surface of the substrate was washed, baked at 200°C. for one hour, and subjected to UV ozone treatment for 370 seconds.

After that, the substrate 1100 was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and subjected to vacuum baking at 170° C. for 30 minutes in a heatingchamber of the vacuum evaporation apparatus, and then the substrate 1100was cooled down for about 30 minutes.

Then, the substrate 1100 over which the anode 1101 was formed was fixedto a substrate holder provided in a vacuum evaporation apparatus so thatthe surface on which the anode 1101 was formed faced downward. Thepressure in the vacuum evaporation apparatus was reduced to about 10⁻⁴Pa. After that, on the anode 1101, by an evaporation method usingresistance heating, 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene)(abbreviation: DBT3P-II) and molybdenum oxide were co-evaporated,whereby a hole-injection layer 1111 was formed. The thickness was set to40 nm, and the weight ratio of DBT3P-II to molybdenum oxide was adjustedto 4:2 (=DBT3P-II: molybdenum oxide). Note that the co-evaporationmethod refers to an evaporation method in which evaporation is carriedout from a plurality of evaporation sources at the same time in onetreatment chamber.

Next, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:BPAFLP) was deposited to a thickness of 20 nm over the hole-injectionlayer 1111, whereby a hole-transport layer 1113 was formed.

Next, 2mDBTBPDBq-II,4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)-triphenylamine(abbreviation: PCBNBB), and(dipivaloylmethanato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)) were co-evaporated to form alight-emitting layer 1115 over the hole-transport layer 1113. Here, theweight ratio of 2mDBTBPDBq-II, PCBNBB, and Ir(tppr)₂(dpm) was adjustedto 0.8:0.2:0.06 (=2mDBTBPDBq-II:PCBNBB:Ir(tppr)₂(dpm)). The thickness ofthe light-emitting layer 1115 was 40 nm.

In the light-emitting layer 1115, 2mDBTBPDBq-II, which is anelectron-transport material, functions as a host material. Further,PCBNBB, which is a hole-transport material, functions as an assistmaterial. Furthermore, Ir(tppr)₂(dpm), which is an organometalliccomplex containing iridium, functions as a guest material.

Further, CzPA was deposited to a thickness of 10 nm over thelight-emitting layer 1115, whereby an electron-transport layer 1117 wasformed.

After that, on the electron-transport layer 1117, bathophenanthroline(abbreviation: BPhen) was deposited to a thickness of 15 nm to form afirst electron-injection layer 1119 a.

Further, lithium fluoride (LiF) was evaporated to a thickness of 1 nm onthe first electron-injection layer 1119 a, so that a secondelectron-injection layer 1119 b was formed.

Lastly, on the second electron-injection layer 1119 b, aluminum wasevaporated to a thickness of 200 nm as a cathode 1103. Thus, thelight-emitting element 1 of this example was formed.

(Comparative Light-Emitting Element 2)

The comparative light-emitting element 2 is different from thelight-emitting element 1 in the electron-transport layer 1117.Specifically, instead of CzPA, which was used in the light-emittingelement 1, 2mDBTBPDBq-II was used for the electron-transport layer 1117of the comparative light-emitting element 2. Note that the thickness of2mDBTBPDBq-II was 10 nm.

The comparative light-emitting element 2 was formed in a manner similarto that of the light-emitting element 1 except the structure of theelectron-transport layer 1117.

(Light-Emitting Element 3)

The light-emitting element 3 is different from the light-emittingelement 1 in the light-emitting layer 1115. Specifically, instead of2mDBTBPDBq-II, PCBNBB, and Ir(tppr)₂(dpm), which were used for thelight-emitting element 1, 2mDBTBPDBq-II, PCBNBB, and(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)) were used for the light-emitting layer1115 of the light-emitting element 3.

The light-emitting layer 1115 of the light-emitting element 3 was formedby co-evaporation of 2mDBTBPDBq-II, PCBNBB, and Ir(tBuppm)₂(acac). Here,the weight ratio of 2mDBTBPDBq-II, PCBNBB and Ir(tBuppm)₂(acac) wasadjusted to 0.8:0.2:0.06 (=2mDBTBPDBq-II:PCBNBB: Ir(tBuppm)₂(acac)). Thethickness of the light-emitting layer 1115 of the light-emitting element3 was 40 nm.

In the light-emitting layer 1115 of the light-emitting element 3,2mDBTBPDBq-II, which is an electron-transport material, functions as ahost material. Further, PCBNBB, which is a hole-transport material,functions as an assist material. Furthermore, Ir(tBuppm)₂(acac), whichis an organometallic complex containing iridium, functions as a guestmaterial.

The components of the light-emitting element 3 other than thelight-emitting layer 1115 were formed in a manner similar to those ofthe light-emitting element 1.

(Comparative Light-Emitting Element 4)

The comparative light-emitting element 4 is different from thelight-emitting element 1 in the light-emitting layer 1115 and theelectron-transport layer 1117. Specifically, instead of 2mDBTBPDBq-II,PCBNBB, and Ir(tppr)₂(dpm), which were used for the light-emittingelement 1, 2mDBTBPDBq-II, PCBNBB, and Ir(tBuppm)₂(acac) were used forthe light-emitting layer 1115 of the comparative light-emitting element4. In addition, instead of CzPA used for the light-emitting element 1,2mDBTBPDBq-II was used for the electron-transport layer 1117 of thecomparative light-emitting element 4.

The light-emitting layer 1115 of the comparative light-emitting element4 was formed by co-evaporation of 2mDBTBPDBq-II, PCBNBB, andIr(tBuppm)₂(acac). Here, the weight ratio of 2mDBTBPDBq-II, PCBNBB andIr(tBuppm)₂(acac) was adjusted to 0.8:0.2:0.06 (=2mDBTBPDBq-II:PCBNBB:Ir(tBuppm)₂(acac)). The thickness of the light-emitting layer 1115 ofthe comparative light-emitting element 4 was 40 nm.

The thickness of the electron-transport layer 1117 of the comparativelight-emitting element 4 was 10 nm.

The components other than the light-emitting layer 1115 and theelectron-transport layer 1117 of the comparative light-emitting element4 were formed in a manner similar to those of the light-emitting element1.

(Light-Emitting Element 5)

The light-emitting element 5 is different from the light-emittingelement 1 in the light-emitting layer 1115. Specifically, instead of2mDBTBPDBq-II, PCBNBB, and Ir(tppr)₂(dpm), which were used for thelight-emitting element 1, CzPA andN,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-pyrene-1,6-diamine(abbreviation 1,6mMemFLPAPrn) were used for the light-emitting layer1115 of the light-emitting element 5.

The light-emitting layer 1115 of the light-emitting element 5 was formedby co-evaporation of CzPA and 1,6mMemFLPAPrn. Here, the weight ratio ofCzPA and 1,6mMemFLPAPrn was adjusted to 1:0.05 (=CzPA:1,6mMemFLPAPrn).The thickness of the light-emitting layer 1115 of the light-emittingelement 5 was 25 nm.

In the light-emitting layer 1115 of the light-emitting element 5, CzPA,which is an electron-transport material, functions as a host material.Further, 1,6mMemFLPAPrn, which is a fluorescent material, functions as aguest material.

The components of the light-emitting element 5 other than thelight-emitting layer 1115 were formed in a manner similar to those ofthe light-emitting element 1.

(Comparative Light-Emitting Element 6)

The comparative light-emitting element 6 is different from thelight-emitting element 1 in the light-emitting layer 1115 and theelectron-transport layer 1117. Specifically, instead of 2mDBTBPDBq-II,PCBNBB, and Ir(tppr)₂(dpm), which were used for the light-emittingelement 1, CzPA and 1,6mMemFLPAPrn were used for the light-emittinglayer 1115 of the comparative light-emitting element 6. In addition,instead of CzPA used for the light-emitting element 1, 2mDBTBPDBq-II wasused for the electron-transport layer 1117 of the comparativelight-emitting element 6.

The light-emitting layer 1115 of the comparative light-emitting element6 was formed by co-evaporation of CzPA and 1,6mMemFLPAPrn. Here, theweight ratio of CzPA and 1,6mMemFLPAPrn was adjusted to 1:0.05(=CzPA:1,6mMemFLPAPrn). The thickness of the light-emitting layer 1115of the comparative light-emitting element 6 was 25 nm.

In the light-emitting layer 1115 of the comparative light-emittingelement 6, CzPA, which is an electron-transport material, functions as ahost material. Further, 1,6mMemFLPAPrn, which is a fluorescent material,functions as a guest material.

The thickness of the electron-transport layer 1117 of the comparativelight-emitting element 6 was 10 nm.

The components other than the light-emitting layer 1115 and theelectron-transport layer 1117 of the comparative light-emitting element6 were formed in a manner similar to those of the light-emitting element1.

The evaporation process of each of the light-emitting elements (thelight-emitting elements 1, 3, and 5) of one embodiment of the presentinvention and the comparative light-emitting elements (the comparativelight-emitting elements 2, 4, and 6) is performed by aresistance-heating method.

As described above, the light-emitting elements (the light-emittingelements 1, 3, and 5) of one embodiment of the present invention and thecomparative light-emitting elements (the comparative light-emittingelements 2, 4, and 6) have the same structure except the structures ofthe light-emitting layer 1115 and the electron-transport layer 1117.

Table 2 shows the element structures of the light-emitting elements (thelight-emitting elements 1, 3, and 5) of one embodiment of the presentinvention and the comparative light-emitting elements (the comparativelight-emitting elements 2, 4, and 6) obtained in the above manner.

TABLE 2 First Second Hole- Hole- Light- Electron- electron- electron-injection transport emitting transport injection injection Anode layerlayer layer layer layer layer Cathode Note Light-emitting ITSODBT3P-II:MoO_(x) BPAFLP See below See below BPhen LiF Al Red-light-element 1 110 nm (=4:2) 20 nm 15 nm 1 nm 200 nm emitting 40 nm elementComparative ITSO DBT3P-II:MoO_(x) BPAFLP See below See below BPhen LiFAl Red-light- light-emitting 110 nm (=4:2) 20 nm 15 nm 1 nm 200 nmemitting element 2 40 nm element Light-emitting ITSO DBT3P-II:MoO_(x)BPAFLP See below See below BPhen LiF Al Green-light- element 3 110 nm(=4:2) 20 nm 15 nm 1 nm 200 nm emitting 40 nm element Comparative ITSODBT3P-II:MoO_(x) BPAFLP See below See below BPhen LiF Al Green-light-light-emitting 110 nm (=4:2) 20 nm 15 nm 1 nm 200 nm emitting element 440 nm element Light-emitting ITSO DBT3P-II:MoO_(x) BPAFLP See below Seebelow BPhen LiF Al Blue-light- element 5 110 nm (=4:2) 20 nm 15 nm 1 nm200 nm emitting 40 nm element Comparative ITSO DBT3P-II:MoO_(x) BPAFLPSee below See below BPhen LiF Al Blue-light- light-emitting 110 nm(=4:2) 20 nm 15 nm 1 nm 200 nm emitting element 6 40 nm elementLight-emitting 2mDBTBPDBq-II:PCBNBB:Ir(tppr)₂dpm CzPA element 1(=0.8:0.2:0.06) 10 nm 40 nm Comparative2mDBTBPDBq-II:PCBNBB:Ir(tppr)₂dpm 2mDBTBPDBq-II light-emitting(=0.8:0.2:0.06) 10 nm element 2 40 nm Light-emitting2mDBTBPDBq-II:PCBNBB:Ir(tBuppm)₂(acac) CzPA element 3 (=0.8:0.2:0.06) 10nm 40 nm Comparative 2mDBTBPDBq-II:PCBNBB:Ir(tBuppm)₂(acac)2mDBTBPDBq-II light-emitting (=0.8:0.2:0.06) 10 nm element 4 40 nmLight-emitting CzPA:1,6mMemFLPAPrn CzPA element 5 (=1:0.05) 10 nm 25 nmComparative CzPA:1,6mMemFLPAPrn 2mDBTBPDBq-II light-emitting (=1:0.05)10 nm element 6 25 nm

As shown in Table 2, as for the light-emitting elements of oneembodiment of the present invention, 2mDBTBPDBq-II is used as hostmaterials of phosphorescent materials (of the light-emitting elements 1and 3) and CzPA is used as a host material of a fluorescent (of thelight-emitting element 5). Further, the same CzPA is used as theelectron-transport layers of the light-emitting elements 1, 3, and 5. Onthe other hand, as for the comparative light-emitting elements,2mDBTBPDBq-II is used as host materials of phosphorescent materials (ofthe comparative light-emitting elements 2 and 4) and CzPA is used as ahost material of a fluorescent material (of the light-emitting element6). The same 2mDBTBPDBq-II is used for the electron-transport layers ofthe comparative light-emitting elements 2, 4, and 6.

Next, in a glove box containing a nitrogen atmosphere, thelight-emitting elements formed as described above were sealed with aglass substrate so as not to be exposed to the air (specifically, asealant was applied onto an outer edge of the element and heat treatmentwas performed at 80° C. for one hour at the time of sealing). Then, theoperation characteristics of the light-emitting elements were measured.It is to be noted that the measurements were performed at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 7 shows current density-luminance characteristics of thelight-emitting element 1 and the comparative light-emitting element 2,FIG. 8 shows voltage-luminance characteristics thereof, FIG. 9 showsluminance-current efficiency characteristics thereof, FIG. 10 showsvoltage-current characteristics thereof, and FIG. 11 shows emissionspectra thereof.

FIG. 12 shows current density-luminance characteristics of thelight-emitting element 3 and the comparative light-emitting element 4,FIG. 13 shows voltage-luminance characteristics thereof, FIG. 14 showsluminance-current efficiency characteristics thereof, FIG. 15 showsvoltage-current characteristics thereof, and FIG. 16 shows emissionspectra thereof.

FIG. 17 shows current density-luminance characteristics of thelight-emitting element 5 and the comparative light-emitting element 6FIG. 18 shows voltage-luminance characteristics thereof, FIG. 19 showsluminance-current efficiency characteristics thereof, FIG. 20 showsvoltage-current characteristics thereof, and FIG. 21 shows emissionspectra thereof.

In FIG. 7 , FIG. 12 , and FIG. 17 , the horizontal axis representscurrent density (mA/cm²), and the vertical axis represents luminance(cd/m²). In FIG. 8 , FIG. 13 , and FIG. 18 , the horizontal axisrepresents voltage (V), and the vertical axis represents luminance(cd/m²). In FIG. 9 , FIG. 14 , and FIG. 19 , the horizontal axisrepresents luminance (cd/m²) and the vertical axis represents currentefficiency (cd/A). In FIG. 10 , FIG. 15 , and FIG. 20 , the horizontalaxis represents voltage (V) and the vertical axis represents current(mA). In FIG. 11 , FIG. 16 , and FIG. 21 , the horizontal axisrepresents wavelength (nm) and the vertical axis represents intensity(arbitrary unit). Note that the emission spectra of the light-emittingelements substantially overlap each other in FIG. 11 , FIG. 16 , andFIG. 21 .

Table 3 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), and externalquantum efficiency (%) of each light-emitting element at a luminance ofaround 1000 cd/m².

TABLE 3 Voltage Current density Luminance Current efficiency externalquantum (V) (mA/cm²) chromaticity x chromaticity y (cd/m²) (cd/A)efficiency (%) Note Light-emitting 3.2 3.6 0.66 0.34 992 27 24Red-light- element 1 emitting element Comparative 3.3 4.0 0.66 0.34 110327 23 Red-light- light-emitting emitting element 2 elementLight-emitting 2.8 0.9 0.43 0.56 804 91 26 Green-light- element 3emitting element Comparative 2.9 1.1 0.43 0.56 987 93 26 Green-light-light-emitting emitting element 4 element Light-emitting 3.2 8.1 0.140.19 905 11 9 Blue-light- element 5 emitting element Comparative 3.5 9.60.14 0.19 1115 12 9 Blue-light- light-emitting emitting element 6element

As shown in Table 3, the light-emitting element 1 having a luminance of992 cd/m² exhibited the following element characteristics: a currentefficiency of 27 cd/A, an external quantum efficiency of 24%, CIEchromaticity coordinates (x, y) of (0.66, 0.34). Further the comparativelight-emitting element 2 having a luminance of 1103 cd/m² exhibited thefollowing element characteristics: a current efficiency of 27 cd/A, anexternal quantum efficiency of 23%, CIE chromaticity coordinates (x, y)of (0.66, 0.34).

Further, as shown in FIG. 11 , the emission spectra of thelight-emitting element 1 and the comparative light-emitting element 2each have a peak at 619 nm.

As described above, the comparison between the light-emitting element 1and the comparative light-emitting element 2 suggests that there is nodifference in element characteristics therebetween. That is, the aboveresults show that the electron-transport layer 1117 (CzPA) of thelight-emitting element 1 and the electron-transport material(2mDBTBPDBq-II) serving as the host material of the phosphorescentmaterial of the light-emitting element 1 each have a highelectron-transport property, and thus the light-emitting element 1 hasan element structure in which light emission excited in thelight-emitting layer 1115 is not diffused or is hardly diffused to theelectron-transport layer 1117 side.

As shown in Table 3, the light-emitting element 3 having a luminance of804 cd/m² exhibited the following element characteristics: a currentefficiency of 91 cd/A, an external quantum efficiency of 26%, CIEchromaticity coordinates (x, y) of (0.43, 0.56). Further the comparativelight-emitting element 4 having a luminance of 987 cd/m² exhibited thefollowing element characteristics: a current efficiency of 93 cd/A, anexternal quantum efficiency of 26%, CIE chromaticity coordinates (x, y)of (0.43, 0.56).

Further, as shown in FIG. 16 , the emission spectra of thelight-emitting element 3 and the comparative light-emitting element 4have a peak at 549 nm and a peak at 546 nm, respectively.

As described above, the comparison between the light-emitting element 3and the comparative light-emitting element 4 suggests that there is nodifference in element characteristics therebetween. That is, the aboveresults show that the electron-transport layer 1117 (CzPA) of thelight-emitting element 3 and the electron-transport material(2mDBTBPDBq-II) serving as the host material of the phosphorescentmaterial of the light-emitting element 3 each have a highelectron-transport property, and thus the light-emitting element 3 hasan element structure in which light emission excited in thelight-emitting layer 1115 is not diffused or is hardly diffused to theelectron-transport layer 1117 side.

As shown in Table 3, the light-emitting element 5 having a luminance of905 cd/m² exhibited the following element characteristics: a currentefficiency of 11 cd/A, an external quantum efficiency of 9%, CIEchromaticity coordinates (x, y) of (0.14, 0.19). Further the comparativelight-emitting element 6 having a luminance of 1115 cd/m² exhibited thefollowing element characteristics: a current efficiency of 12 cd/A, anexternal quantum efficiency of 9%, CIE chromaticity coordinates (x, y)of (0.14, 0.19).

Further, as shown in FIG. 21 , the emission spectra of thelight-emitting element 5 and the comparative light-emitting element 6have a peak at 464 nm and a peak at 465 nm, respectively.

As described above, the comparison between the light-emitting element 5and the comparative light-emitting element 6 suggests that there is adifference in element characteristics therebetween. Specifically, asshown in Table 3 and FIG. 20 , they mainly differ in voltage-currentcharacteristics. The light-emitting element 5 has a voltage of 3.3 V at905 cd/m², and the comparative light-emitting element 6 has a voltage of3.5 V at 1115 cd/m². Further, as shown in FIG. 20 , when the voltageincreases from around 3 V, the comparative light-emitting element 6 hasa current value lower than that of the light-emitting element 5 of oneembodiment of the present invention.

This is because the electron-transport layer 1117 of the comparativelight-emitting element 6 includes the electron-transport material(2mDBTBPDBq-II) serving as the host material of the phosphorescentmaterial. The host material of the phosphorescent material has a lowerelectron-transport property than the electron-transport material (CzPA)used for the light-emitting layer 1115.

On the other hand, in the light-emitting element 5 of one embodiment ofthe present invention, the electron-transport layer 1117 (CzPA) has abetter electron-transport property than the electron-transport material(2mDBTBPDBq-II) serving as the host material of the phosphorescentmaterial; therefore, the light-emitting element 5 has more favorableelement characteristics with lower driving voltage.

Note that the structure described in this example can be combined asappropriate with any of the structures described in the embodiments orthe other examples.

Example 3

In this example, the light-emitting elements (the light-emittingelements 7 and 8) of one embodiment of the present invention aredescribed with reference to FIG. 6B. Chemical formulae of materials usedin this example are shown below.

Methods for manufacturing the light-emitting elements (thelight-emitting elements 7 and 8) of one embodiment of the presentinvention, which are used in this example, are described below.

Note that the light-emitting element 7 is a red-light-emitting element,and the light-emitting element 8 is a green-light-emitting element.

(Light-Emitting Element 7)

First, over the substrate 1100, an indium oxide-tin oxide compoundcontaining silicon or silicon oxide (ITSO) was deposited by a sputteringmethod, so that the anode 1101 was formed. Note that the compositionratio of In₂O₃ to SnO₂ and SiO₂ in the target used was 85:10:5 [wt %].The thickness of the anode 1101 was 110 nm and the electrode area was 2mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, the surface of the substrate was washed, baked at 200°C. for one hour, and subjected to UV ozone treatment for 370 seconds.

After that, the substrate 1100 was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and subjected to vacuum baking at 170° C. for 30 minutes in a heatingchamber of the vacuum evaporation apparatus, and then the substrate 1100was cooled down for about 30 minutes.

Then, the substrate 1100 over which the anode 1101 was formed was fixedto a substrate holder provided in a vacuum evaporation apparatus so thatthe surface on which the anode 1101 was formed faced downward. Thepressure in the vacuum evaporation apparatus was reduced to about 10⁻⁴Pa. After that, on the anode 1101, by an evaporation method usingresistance heating, 4,4′,4″-(benz ene-1,3,5-triyl)tri(dibenzothiophene)(abbreviation: DBT3P-II) and molybdenum oxide were deposited byco-evaporation, whereby the hole-injection layer 1111 was formed. Thethickness was set to 40 nm, and the weight ratio of DBT3P-II tomolybdenum oxide was adjusted to 4:2 (=DBT3P-II:molybdenum oxide).

Next, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:BPAFLP) was deposited to a thickness of 20 nm over the hole-injectionlayer 1111, whereby the hole-transport layer 1113 was formed.

Next, 2mDBTBPDBq-II,4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)-triphenylamine(abbreviation: PCBNBB), and(dipivaloylmethanato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(dpm)) were co-evaporated to form alight-emitting layer 1115 over the hole-transport layer 1113. Here, theweight ratio of 2mDBTBPDBq-II, PCBNBB, and Ir(tppr)₂(dpm) was adjustedto 0.8:0.2:0.06 (=2mDBTBPDBq-II:PCBNBB:Ir(tppr)₂(dpm)). The thickness ofthe light-emitting layer 1115 was 40 nm.

In the light-emitting layer 1115, 2mDBTBPDBq-II, which is anelectron-transport material, functions as a host material. Further,PCBNBB, which is a hole-transport material, functions as an assistmaterial. Furthermore, Ir(tppr)₂(dpm), which is an organometalliccomplex containing iridium, functions as a guest material.

Further, CzPA and 1,6mMemFLPAPrn were co-evaporated over thelight-emitting layer 1115, whereby an electron-transport layer 1117 awas formed over the light-emitting layer 1115. Here, the weight ratio ofCzPA to 1,6mMemFLPAPrn was adjusted to 1:0.05 (=CzPA:1,6mMemFLPAPrn).The thickness of the electron-transport layer 1117 a of thelight-emitting element 7 was 25 nm.

Note that the electron-transport layer 1117 a of the light-emittingelement 7 has the same structure as the light-emitting layers of thelight-emitting element 5 and the comparative light-emitting element 6described in Example 2. That is, a blue-light-emitting layer is used asthe electron-transport layer 1117 a of the light-emitting element 7.

After that, on the electron-transport layer 1117, bathophenanthroline(abbreviation: BPhen) was deposited to a thickness of 15 nm to form thefirst electron-injection layer 1119 a.

Further, lithium fluoride (LiF) was evaporated to a thickness of 1 nm onthe first electron-injection layer 1119 a, so that the secondelectron-injection layer 1119 b was formed.

Lastly, on the second electron-injection layer 1119 b, aluminum wasevaporated to a thickness of 200 nm as the cathode 1103. Thus, thelight-emitting element 7 of this example was formed.

(Light-Emitting Element 8)

The light-emitting element 8 is different from the light-emittingelement 7 in the light-emitting layer 1115. Specifically, instead of2mDBTBPDBq-II, PCBNBB, and Ir(tppr)₂(dpm), which were used for thelight-emitting element 7, 2mDBTBPDBq-II, PCBNBB, and Ir(tBuppm)₂(acac)were used for the light-emitting layer 1115 of the light-emittingelement 8.

The light-emitting layer 1115 of the light-emitting element 8 was formedover the hole-transport layer 1113 by co-evaporation of 2mDBTBPDBq-II,PCBNBB, and Ir(tBuppm)₂(acac). Here, the weight ratio of 2mDBTBPDBq-II,PCBNBB and Ir(tBuppm)₂(acac) was adjusted to 0.8:0.2:0.06(=2mDBTBPDBq-II:PCBNBB: Ir(tBuppm)₂(acac)). The thickness of thelight-emitting layer 1115 of the light-emitting element 8 was 40 nm.

In the light-emitting layer 1115 of the light-emitting element 8,2mDBTBPDBq-II, which is an electron-transport material, functions as ahost material. Further, PCBNBB, which is a hole-transport material,functions as an assist material. Furthermore, Ir(tBuppm)₂(acac), whichis an organometallic complex containing iridium, functions as a guestmaterial.

The electron-transport layer 1117 a of the light-emitting element 8 hasthe same structure as the light-emitting layers of the light-emittingelement 5 and the comparative light-emitting element 6 described inExample 2 in a manner similar to that of the light-emitting element 7.That is, a blue-light-emitting layer is used for the electron-transportlayer 1117 a of the light-emitting element 8.

The components of the light-emitting element 8 other than thelight-emitting layer 1115 were formed in a manner similar to those ofthe light-emitting element 7.

The evaporation process of each of the light-emitting elements (thelight-emitting elements 7 and 8) of one embodiment of the presentinvention is performed by a resistance-heating method.

Table 4 shows element structures of the thus obtained light-emittingelements (the light-emitting elements 7 and 8).

TABLE 4 First Second Hole- Hole- Light- Electron- electron- electron-injection transport emitting transport injection injection Anode layerlayer layer layer layer layer Cathode Note Light- ITSO DBT3P-II:MoO_(x)BPAFLP See below See below BPhen LiF Al Red-light- emitting 110 nm(=4:2) 20 nm 15 nm 1 nm 200 nm emitting element 7 40 nm element Light-ITSO DBT3P-II:MoO_(x) BPAFLP See below See below BPhen LiF AlGreen-light- emitting 110 nm (=4:2) 20 nm 15 nm 1 nm 200 nm emittingelement 8 40 nm element Light- 2mDBTBPDBq-II:PCBNBB:Ir(tppr)₂dpmCzPA:1,6mMemFLPAPrn emitting (=0.8:0.2:0.06) (=1:0.05) element 7 40 nm25 nm Light- 2mDBTBPDBq-II:PCBNBB:Ir(tBuppm)₂(acac) CzPA:1,6mMemFLPAPrnemitting (=0.8:0.2:0.06) (=1:0.05) element 8 40 nm 25 nm

As shown in Table 4, as for the light-emitting elements of oneembodiment of the present invention, 2mDBTBPDBq-II is used as hostmaterials of phosphorescent materials (of the light-emitting elements 7and 8). Moreover, the same CzPA and 1,6mMemFLPAPrn are used for theelectron-transport layers of the light-emitting elements 7 and 8.

Next, in a glove box containing a nitrogen atmosphere, thelight-emitting elements formed as described above were sealed with aglass substrate so as not to be exposed to the air (specifically, asealant was applied onto an outer edge of the element and heat treatmentwas performed at 80° C. for one hour at the time of sealing). Then, theoperation characteristics of the light-emitting elements were measured.It is to be noted that the measurements were performed at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 22 shows current density-luminance characteristics of thelight-emitting elements 7 and 8, FIG. 23 shows voltage-luminancecharacteristics thereof, FIG. 24 shows luminance-current efficiencycharacteristics thereof, FIG. 25 shows voltage-current characteristicsthereof, and FIG. 26 shows emission spectra thereof.

In FIG. 22 , the horizontal axis represents current density (mA/cm²),and the vertical axis represents luminance (cd/m²). In FIG. 23 , thehorizontal axis represents voltage (V), and the vertical axis representsluminance (cd/m²). In FIG. 24 , the horizontal axis represents luminance(cd/m²) and the vertical axis represents current efficiency (cd/A). InFIG. 25 , the horizontal axis represents voltage (V) and the verticalaxis represents current (mA). In FIG. 26 , the horizontal axisrepresents wavelength (nm) and the vertical axis represents intensity(arbitrary unit).

Further, Table 5 shows the voltage (V), current density (mA/cm²), CIEchromaticity coordinates (x, y), current efficiency (cd/A), and externalquantum efficiency (%) of each light-emitting element at a luminance ofaround 1000 cd/m².

TABLE 5 Current Current external Voltage density Luminance efficiencyquantum (V) (mA/cm²) chromaticity x chromaticity y (cd/m²) (cd/A)efficiency (%) Note Light- 3.3 3.6 0.66 0.34 984 27 25 Red-light-emitting emitting element 7 element Light- 3.0 1.2 0.44 0.56 948 76 23Green-light- emitting emitting element 8 element

As shown in Table 5, the light-emitting element 7 having a luminance of984 cd/m² exhibited the following element characteristics: a currentefficiency of 27 cd/A, an external quantum efficiency of 25%, CIEchromaticity coordinates (x, y) of (0.66, 0.34). Further thelight-emitting element 8 having a luminance of 948 cd/m² exhibited thefollowing element characteristics: a current efficiency of 76 cd/A, anexternal quantum efficiency of 23%, CIE chromaticity coordinates (x, y)of (0.44, 0.56).

As shown in FIG. 26 , the emission spectrum of the light-emittingelement 7 has a peak at 620 nm and the light-emitting element 8 has apeak at 548 nm. Further, FIG. shows that blue light emission from1,6mMemFLPAPrn used in the electron-transport layer (see FIG. 21 ) isnot observed.

As described above, the element characteristics equivalent to those ofthe light emitting element 1 described in Example 2 can be obtained inthe light-emitting element 7 of one embodiment of the present inventioneven when a blue-light-emitting layer is used as the electron-transportlayer 1117 a. Further, the element characteristics equivalent to thoseof the light emitting element 3 described in Example 2 can be obtainedin the light-emitting element 8 of one embodiment of the presentinvention even when a blue-light-emitting layer is used as theelectron-transport layer 1117 a.

Therefore, since the host material (CzPA) of the fluorescent materialused for the electron-transport layer 1117 a and the electron-transportmaterial (2mDBTBPDBq-II) serving as the host material of thephosphorescent material each have a high electron-transport property,the light-emission region of the light-emitting element is formed in thelight-emitting layer 1115 in the vicinity of the hole-transport layer1113, and light emission excited in the light-emitting layer 1115 is notdiffused or is hardly diffused to the electron-transport layer 1117 aside. The electron-transport layer 1117 a used in the light-emittingelements 7 and 8 includes 1,6mMemFLPAPrn, which is a fluorescentmaterial. However, as shown in FIGS. 22 to 26 , 1,6mMemFLPAPrn, which isa fluorescent material, does not affect the element characteristics.

Note that the structure described in this example can be combined asappropriate with any of the structures described in the embodiments orthe other examples.

Example 4

In this example, the light-emitting elements 1, 3, 7, and 8, which arethe light-emitting elements of one embodiment of the present inventionand formed in Examples 2 and 3, and the comparative light-emittingelements 2 and 4, which are light-emitting elements for comparison andformed in Examples 2 and 3, are subjected to reliability tests. Theresults of the reliability tests are shown in FIGS. 27A and 27B.

FIG. 27A shows the results of reliability tests of the light-emittingelement 1, the comparative light-emitting element 2, and thelight-emitting element 7, i.e., red-light-emitting elements. FIG. 27Bshows the results of reliability tests of the light-emitting element 3,the comparative light-emitting element 4, and the light-emitting element8, i.e., green-light-emitting elements. In the reliability test, eachlight-emitting element was driven under the conditions where the initialluminance was 5000 cd/m² and the current density was constant. Theresults are shown in FIGS. 27A and 27B. The horizontal axis representsdriving time (h) of the element and the vertical axis representsnormalized luminance (%) on the assumption that the initial luminance is100%. In FIGS. 27A and 27B, data of the light-emitting elementssubstantially overlap with each other.

The results of FIG. 27A show that normalized luminance of thelight-emitting element 1 obtained after 357 hours was 68%. Further,normalized luminance of the comparative light-emitting element 2obtained after 357 hours was 68%. Furthermore, normalized luminance ofthe light-emitting element 7 obtained after 357 hours was 66%. FIG. 27Bshows that normalized luminance of the light-emitting element 3 obtainedafter 688 hours was 81%. Further, normalized luminance of thecomparative light-emitting element 4 obtained after 688 hours was 82%.Furthermore, normalized luminance of the light-emitting element 8obtained after 688 hours was 80%.

As described above, the results of the reliability tests of thelight-emitting elements 1 and 7 that are embodiments of the presentinvention are equivalent to the result of the reliability test of thecomparative light-emitting element 2. Further, the results of thereliability tests of the light-emitting elements 3 and 8 that areembodiments of the present invention are equivalent to the result of thereliability test of the comparative light-emitting element 4.

Note that the structure described in this example can be combined asappropriate with any of the structures described in the embodiments orthe other examples.

This application is based on Japanese Patent Application serial No.2013-065394 filed with Japan Patent Office on Mar. 27, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting device comprising: a firstlight-emitting element comprising a first light-emitting layer; and asecond light-emitting element comprising a second light-emitting layer,wherein the first light-emitting element and the second light-emittingelement comprise an electron-transport layer, wherein the firstlight-emitting layer comprises a first phosphorescent material, a firstorganic material and a second organic material, wherein the secondlight-emitting layer comprises a fluorescent material and a thirdorganic material, wherein the electron-transport layer comprises afourth organic material, wherein the first light-emitting layer and thesecond light-emitting layer are each in contact with theelectron-transport layer positioned on a cathode side, wherein a tripletexcitation energy of the fourth organic material is lower than a tripletexcitation energy of the first organic material, wherein the firstorganic material and the second organic material form an exciplex,wherein the second organic material comprises a carbazole skeleton, andwherein the fourth organic material comprises a first anthracenederivative.
 2. The light-emitting device according to claim 1, furthercomprising a third light-emitting element comprising a thirdlight-emitting layer, wherein the third light-emitting layer comprises asecond phosphorescent material and a fifth organic material, wherein thethird light-emitting layer is in contact with the electron-transportlayer positioned on the cathode side, and wherein the triplet excitationenergy of the fourth organic material is lower than a triplet excitationenergy of the fifth organic material.
 3. The light-emitting deviceaccording to claim 1, wherein the third organic material comprises asecond anthracene derivative.
 4. The light-emitting device according toclaim 1, wherein the electron-transport layer comprises a sixth organicmaterial.
 5. The light-emitting device according to claim 1, wherein thefirst phosphorescent material is an organometallic complex containingiridium.
 6. The light-emitting device according to claim 1, wherein thefirst light-emitting layer emits red or green light, and wherein thesecond light-emitting layer emits blue light.
 7. The light-emittingdevice according to claim 2, wherein the second phosphorescent materialis an organometallic complex containing iridium.
 8. The light-emittingdevice according to claim 2, wherein the first light-emitting layeremits red light, wherein the second light-emitting layer emits bluelight, and wherein the third light-emitting layer emits green light. 9.An electronic appliance comprising the light-emitting device accordingto claim
 1. 10. A lighting device comprising the light-emitting deviceaccording to claim
 1. 11. A light-emitting device comprising: a firstlight-emitting element comprising a first light-emitting layer; and asecond light-emitting element comprising a second light-emitting layer,wherein the first light-emitting element and the second light-emittingelement comprise an electron-transport layer, wherein the firstlight-emitting layer comprises a first phosphorescent material, a firstorganic material and a second organic material, wherein the secondlight-emitting layer comprises a fluorescent material and a thirdorganic material, wherein the electron-transport layer comprises afourth organic material, wherein a triplet excitation energy of thefourth organic material is lower than a triplet excitation energy of thefirst organic material, wherein the first organic material and thesecond organic material form an exciplex, wherein an emission spectrumof the exciplex overlaps with an absorption band on the longestwavelength side in an absorption spectrum of the first phosphorescentmaterial, wherein the second organic material comprises a carbazoleskeleton, and wherein the fourth organic material comprises a firstanthracene derivative.
 12. The light-emitting device according to claim11, further comprising a third light-emitting element comprising a thirdlight-emitting layer, wherein the third light-emitting layer comprises asecond phosphorescent material and a fifth organic material, and whereinthe triplet excitation energy of the fourth organic material is lowerthan a triplet excitation energy of the fifth organic material.
 13. Thelight-emitting device according to claim 11, wherein the firstphosphorescent material is an organometallic complex containing iridium.14. The light-emitting device according to claim 11, wherein the firstlight-emitting layer emits red or green light, and wherein the secondlight-emitting layer emits blue light.
 15. The light-emitting deviceaccording to claim 12, wherein the second phosphorescent material is anorganometallic complex containing iridium.
 16. The light-emitting deviceaccording to claim 12, wherein the first light-emitting layer emits redlight, wherein the second light-emitting layer emits blue light, andwherein the third light-emitting layer emits green light.
 17. Anelectronic appliance comprising the light-emitting device according toclaim
 11. 18. A lighting device comprising the light-emitting deviceaccording to claim
 11. 19. The light-emitting device according to claim11, wherein the first light-emitting layer and the second light-emittinglayer are each in contact with the electron-transport layer positionedon a cathode side.